Infection of rat fibroblasts with early mutants of polyoma virus (tsa) or simian virus 40 (tsA30) leads to the establishment of either temperature-independent A transformants or N transformants temperature-sensitive for the expression of the transformed phenotype. The choice between the A- and N-transformed states is not only dependent, as we reported previously (Rassoulzadegan et al., j. Virol., 28:421-426, 1978), on the growth conditions after infection, but is also a function of the multiplicity of infection (MOI); high MOI led to the predominant occurrence of A derivatives, and lower MOI led to that of N transformants.