Various aspects of the control by serum and nutrients on the cell cycle in normal and transformed cells are discussed. The serum factor requirement of untransformed 3T3 mouse fibroblasts is different from that of simian virus 40-transformed 3T3 (SV-3T3) cells. Chinese hamster embryo cells transformed by polyoma virus ts-3 mutant (Py-ts-3-CHE) growing at permissive or nonpermissive temperature have different requirements for nutrients. The lengths of Gl, S, and G2 in SV-3T3 cells vary with the growth rate; in 3T3 cells the lengths of Sand G2 are constant and independent of the growth rate, whereas G1 varies widely. Density-inhibited HeLa cells in suspension are arrested in G0 but start growing again when cells are plated in plastic tissue culture dishes in the depleted spinner medium, which suggests that membrane alterations resulting from cell attachment could lead to the initiation in the growth cycle.