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The successful use of the fluoropyrimidines to gain information about the synthesis of viral DNA (1-3) led us to a study of their effect on the replication of vacuolating virus (SV40). This agent belongs to the papovavirus group 1 (4) which is characterized by 40 to 50 m/~ icosahedral particles possessing a shell of 42 capsomeres and a core of DNA (5, 6). As the time sequence of virus production in the one-step growth cycle had already been determined for SV40 (7), experiments could be readily designed with 5-fluorouracil (FU) and 5-fluorodeoxyuridine (FUDR) to determine whether the synthesis of infectious virus might be suppressed while the formation of viral proteins continued. Reissig and Kaplan (1) had shown that with another icosahedral DNAcontaining virus (swine herpes virus or pseudorabies) grown in the presence of FU, viral protein was assembled into empty particles having the size and shape of herpes virus but without DNA cores. In view of the well known tumorigenic activity of the papovaviruses, and the recent finding that SV40-induced tumors and transformed cells may contain complement-fixing viral antigen in the absence of infectious virus (8), information on the formation of viral antigens in cells which fail to make infectious virus is highly desirable. Materials and MethodsVirus.--The same frozen stock of virus that had been used for determining the growth cycle of SV40 (7) was employed. Its titer was 10 ~'5 plaque-forming units (PFU) per ml.