Mitotic blocking agents, colchicine or Velban, were used to estimate cycle times of spleen cells which release hemolysin for sheep erythrocytes (plaque-forming cells). The cells were obtained either from rats immunized with sheep erythrocytes or from cultures of mouse spleen cells immunized in vitro with the same antigen. 2, 3, or 4 days after immunization, animals or cell cultures were treated with mitotic blocking agents for periods of time ranging from 2.5 to 7 hr; plaque-forming cells were then enumerated. Decreased numbers of plaque-forming cells were found after such treatment. The extent of reduction was a function of duration of the drug treatment and the method of immunization, but was independent of the time after immunization. The evidence presented is consistent with premises that: (a) plaque-forming cells in mitosis do not release sufficient antibody to be detected, (b) mitotic blocking agents, by arresting plaque-forming cells in metaphase, prevent not only detection of these cells but also the increase in number of plaque-forming cells which would have resulted from cell division, (c) mitotic blocking agents do not affect release of antibody by cells in interphase. Cell cycle times, based on the extent of reduction of plaque-forming cells per unit time of drug treatment, were estimated using a mathematical model appropriate for an exponentially increasing population of cells. Cell cycle times estimated using the mitotic blocking agents agreed well with cell doubling times calculated from the increase in plaque-forming cells occurring 1-4 days after immunization. Increased responses produced by higher antigen doses or treatment of immunized animals with an adjuvant resulted from an increased rate of division of responding cells and their progeny. The results are consistent with a cell selection theory of antibody formation. Antigenic stimulation causes relatively few cells to proliferate and to synthesize antibody; apparently the magnitude of the response is dependent primarily on the rate of division of responding cells. It is suggested on the basis of observations of in vitro-immunized cell cultures that the rate of division of responding cells may be dependent on the rate of interaction between two cell types, both of which are essential for the in vitro plaque-forming cell response.