Previous experiments have shown that in the cultured lens the serum-induced stimulation of DNA synthesis and mitosis, which begins at about 24 h and 40 h of culture, respectively, is not dependent on the presence of whole serum after the first 20 h of culture. The present report demonstrates that during approximately the first 4 h of culture, the stimulating mechanism is also independent of the presence of whole serum. The results are discussed in terms of the compartmentalization of the period prior to DNA synthesis into three sub-phases. Each subphase is characterized by its requirement or lack of requirement for the presence of whole serum, or some component (S) of whole serum. The possible significance of the subphases is discussed.