BP3T3, a clonal benzo(a)pyrene-transformed BALB/c-3T3 cell line, is conditionally responsive to growth factor stimulation. Density arrested cell populations deprived of growth factors by pretreatment with 0.5% platelet-poor plasma synthesized DNA both in response to ng/ml concentrations of PDGF, EGF, and somatomedin C, and in response to insulin, plasma, and serum. The above agents acted singly to induce DNA synthesis, but synergism is suggested because a higher percentage of cells were stimulated to enter the S phase when the growth factors were added in combination. Desensitization to growth factors occurred when cultures were pretreated with the high concentration of growth factors present in 10% serum (or plasma). In desensitized cultures none of the above agents, added singly or in combination, stimulated DNA synthesis. This effect appears to be global because pretreatment with one growth factor (e.g., insulin) inhibited the action of another (e.g., PDGF). Cell density appears to play a critical role in regulating DNA synthesis. Unlike nontransformed BALB/c-3T3 cells whose density is regulated by the serum concentration, the density of BP3T3 cells reached a plateau when cultures were grown in a serum (or plasma) concentration of 3% or greater. Such density arrested cultures were growth factor unresponsive; however, the cells rapidly responded to growth factors by synthesizing DNA and replicating when reseeded at a lower cell density. Thus the growth of BP3T3 cells is regulated by both growth factors and cell density.