Patients with IgG multiple myeloma underwent serial studies of tumor cell kinetics including (1) estimation of the total body myeloma cell number (TBMC), (2) measurement of the myeloma cell tritiated thymidine labeling index (LI), and (3) calculation of the total number of myeloma cells undergoing DNA synthesis. Intermittent courses of chemotherapy with cycle-non-specific agents such as melphalan resulted in a marked increase in the LI of myeloma cells in patients who had a 75% reduction in TBMC. The long "plateau" phase of partial remission of myeloma in these patients was associated with a continued high LI: this suggests that the plateau resulted from a balance between the cytoreductive effects of chemotherapy and expansion of the growth fraction (GF) of the tumor. Preliminary attempts to capitalize therapeutically on this expansion of the GF in several patients included administration of the cycle-active agents vincristine and cytosine arabinoside. Vincristine appeared to induce a further reduction in tumor in several patients, although cytosine arabinoside appeared to be ineffective despite clear evidence of its inhibition of DNA synthesis in myeloma cells in vivo. Further clinical studies of the effects of cycle-active drugs on myeloma appear to be warranted; however, successful exploitation of the dynamic change in myeloma cell kinetics with chemotherapy will require the use of cycle-active agents with marked selective toxicity for myeloma cells.