Sequential sera from 45 patients with multiple myeloma (MM) and from 6 patients with solid tumors but normal bone marrows who received cyclophosphamide, 15 mg/kg/day for 4 days, were assayed for their effects on tritiated thymidine (3H-TdR) incorporation by normal bone marrow cells and malignant plasma cells. Pretreatment sera from 23 of the 45 patients with MM inhibited normal marrow cell proliferation relative to the effects of normal sera. Of these 45 sera, 30 inhibited plasma cell proliferation. This humoral inhibition was overcome by the induction of humoral stimulation at a predictable time during chemotherapy. The sera obtained sequentially from patients with MM and patients with normal bone marrows increased 3H-TdR uptake by both cell types by days 12–15 of therapy. Sequential changes in malignant marrow plasma cell 3H-TdR labeling indices paralleled the changes in serum activity, with an increased tumor cell growth fraction occurring at the time of peak serum stimulatory activity. The relationship between serum stimulation and malignant plasma cell proliferation was confirmed in vitro.