Human B lymphocytes activated by pokeweed mitogen (PWM) undergo a proliferative response, differentiate into immunoglobulin producing cells (IPC), and release immunoglobulin into the supernatant fluid of the lymphocyte cultures. Since the number of IPC and amount of immunoglobulin produced may be determined in part by the proportions of T and non-T lymphocytes in the culture, studies were undertaken on the kinetics of the PWM induced response in cultures containing 1 : 1 proportions of T and non-T cells from nine healthy adults. Blood samples were obtained from each individual on two occasions and the results compared. The proliferative response, number of cells containing intracytoplasmic immunoglobulin (ICIg) and the concentration of supernatant immunoglobulin (SIg) were serially determined. The mean peak proliferative response and the mean peak number of ICIg positive cells were observed on the fifth day of culture. The mean peak concentration of SIg was noted on the ninth to eleventh day of culture. The results of the initial specimens were not significantly different from those of the repeat samples. It is concluded that a 1 : 1 mixture of T and non-T cells respond reproducibly to PWM and that the level of response varies with the length of incubation of the cultures. It is believed these assays might be helpful in the investigation of various lymphoproliferative and immunodeficiency disorders.