Early viral polypeptides synthesized in simian virus 40 rat transformants were identified by immunoprecipitation using anti-T (tumor) antigen immune serum. Four polypeptide classes could be identified, which were not detectable in extracts of nontransformed cells and were not precipitated from transformed cell extracts by nonimmune serum. Their apparent M(r) were 92,000, 63,000, 56,000, and 19,000. A similar pattern was observed in extracts from lytically infected cells, but the relative rate of radioactive labeling of the M(r) 63,000 and 56,000 species was in this case significantly lower than in transformed cells. In tsA30 transformants of type A, which maintain the transformed phenotype at high temperature, only minor quantitative variations of this pattern were observed when the cultures were shifted from 33 degrees to 40.5 degrees . In contrast, the rate of labeling of the four virus-specific polypeptides was decreased by 90% or more at high temperature in the temperature-sensitive N transformants. In all cases, a coordinated variation of the radioactivity associated with the different polypeptide classes was observed. These results suggest that the synthesis or processing, or both, of the viral early proteins may be controlled by different mechanisms in various types of simian virus 40 transformants and, furthermore, that it may be under the positive control of a virus-coded protein in transformed cells of type N.