Bone marrow cell responses to injections of nitrogen mustard, oncovin, procarbazine, hydrocortisone, and a regimen of all four drugs (MOPH) were evaluated in CRF1 and C57B1/6 mice by determining bone marrow cellularity and content of transplantable colony forming units (CFU) after treatment. The study was done to determine whether the combined regimen, which is widely used clinically in treatment of disseminated Hodgkin's disease, is more or less detrimental to the hemopoietic system than the same drugs used as single agents. Nitrogen mustard and procarbazine used clinically as single drugs are given in three and two times, respectively, greater doses than in the combined regimen. Hydrocortisone, given singly, was least toxic of the drugs, reducing the CFU/femur to 63% and 71% of control values. MOPH appeared slightly more toxic than hydrocortisone, resulting in 41% and 52% of the CFU/femur surviving, and was about equally as toxic as oncovin alone. Nitrogen mustard and procarbazine, administered as single drugs in high doses, were highly suppressive, resulting in only 10-19% survival of CFU/femur, whereas, reduced doses of the two drugs as used in the MOPH regimen spared 30-45% of the CFU/femur. Survival of CFU after MOPH treatment was three to four times greater than after high doses of nitrogen mustard or procarbazine alone. The component drugs of the combined regimen did not act on separate populations of stem cells to produce an additive effect but appeared to inactivate the same population of cells.