A study of 32 patients receiving cyclophosphamide (CY) and verapamil (VER) in addition to the drug combination vincristine, adriamycin and methyl prednisolone (VAMP) was made in which the clinical response and growth of clonogenic myeloma cells (MY-CFUc) from bone marrow aspirates were compared. At presentation, MY-CFUc were grown from 72 per cent (23/32) of the patients. After treatment with CY-VAMP or VERCY-VAMP, MY-CFUc were grown from 25 per cent (8/32) of patients of whom only 50 per cent responded clinically. The overall clinical response rate for patients receiving CY-VAMP and VERCY-VAMP was 64 per cent (9/14) and 72 per cent (13/18) respectively of whom 14 per cent in each group achieved complete remission. There was no concomitant increase in normal tissue toxicity as measured by granulocyte-macrophage colony (GM-CFUc) formation. Comparison of these data with our previous study of patients receiving VAMP alone, suggests that the addition of CY to the regimen may increase the tumour cell kill. Further clinical studies will determine whether there is a significant increase in the complete remission rate.