The early gene of wild-type (wt) SV40 specifies two related proteins, referred to as large (Mr 88000) and small (Mr 19000) T-antigen. Infection with wt SV40 of Go/Gl-arrested monkey kidney and CV-1 cell cultures induced in virtually 100% of the cells T-antigen synthesis, followed by a mitotic reaction and the production of SV40 DNA. Parallel cultures were infected with SV40 deletion mutants that produce either no small T-antigen (dl883) or only trace amounts of a truncated form (dl891). Kinetics of synthesis and accumulation of large T-antigen was closely similar to that observed with wtSV40 whereas apparently only 50–60% of the cells participated in the mitotic reaction and the production of viral DNA. These results and those obtained from a comparative study on the abortive (transforming) infection in Go-arrested mouse tissue culture cells indicate that synthesis of large T-antigen alone is sufficient to trigger in 50–60% of the infected cells a mitotic reaction.