After his graduation from the Stern Conservatory, Kroyt embarked on an international concert career as a violinist, playing in solo recitals and violin concertos, and with
string quartets. He played as a violin soloist under the conductors
Richard Strauss and
Erich Kleiber and in chamber ensembles with the cellist
Pablo Casals and the pianists
Artur Schnabel, and
Artur Rubenstein. He founded his own string quartet in 1921 and from 1927 also played with the original Guarneri Quartet for seven years as its violist. He had learned the viola as a teenager in three days when Alexander Fiedemann, who had his own string quartet at the time, insisted that Kroyt substitute for the quartet's violist who had taken ill. In 1924 Kroyt played in the first post-World War I performance of
Schoenberg's
Pierrot Lunaire in an ensemble that included Schnabel, the cellist
Gregor Piatigorsky and the soprano
Marie Gutheil-Schoder. A loud disturbance with boos and shrieking from anti-modernists broke out as the performance began. The music theorist Fritz-Fridolin Windisch jumped onto the stage to continue the protests and had to be forcibly removed. At that point Schnabel, Kroyt, and Piatigorsky began playing a circus polka. The audience burst out laughing, after which the performance went on without interruption to a successful reception. To supplement his meager earnings as a classical musician in the 1920s, Kroyt also played jazz in the Ruscho and Tariffa cafés in Berlin and led a small orchestra that performed and recorded
tango music and
operetta tunes. Unwilling to have his real name associated with the orchestra, he appeared with them as "Tino Valerio". In 1932 Kroyt married Sophie (Sonya) Blumin. Born in Lithuania in 1908, she was the daughter of a wealthy Jewish architect. The family had settled in Moscow but fled Russia for Germany during the
Bolshevik Revolution. She was a ballet dancer by training and had also studied law at the
University of Berlin. Their daughter and only child, Yanna, was born the following year. Life was becoming increasingly difficult for Jewish musicians under Nazi Germany. In Germany itself, Kroyt could only play for Jewish organizations and he would need to concentrate on foreign engagements to support his family. In May 1936 he accepted an offer to play in an orchestra in Tel Aviv that was being formed by
William Steinberg and
Bronisław Huberman. However, later that month his old friend Josef Roisman (1900–1974), who was the First Violin for the
Budapest String Quartet, asked him to replace their recently resigned violist . Reluctant to move his young family to
Palestine and an uncertain future and seeing the large number of international concerts for which the quartet were contracted, Kroyt accepted Roisman's offer. He played his first concert with the quartet on 31 August 1936 in Norway and would remain their violist until the ensemble disbanded in 1967. The Budapest String Quartet were in the United States when World War II broke out in Europe. They accepted an offer from the
Library of Congress to become resident there, playing on the
Stradivarius string instruments in the library's collection in an annual series of 20 concerts at the
Elizabeth Sprague Coolidge Auditorium. The Kroyt family settled in a house in
Northwest Washington, D.C. where they became known for their soirées which were attended by prominent musicians and political figures. Kroyt and his wife and daughter became naturalized U.S. citizens in 1944 and lived in the United States for the rest of their lives. Kroyt owned and played a
Deconet viola. When the Budapest String Quartet took up residence at the Library of Congress, the library additionally loaned Stradivarius instruments to all its members. ==Later years==