Human peripheral blood mononuclear cells (PBMC) were stimulated with pokeweed mitogen (PWM) for 3 days and subsequently cultured in medium for another 3 days. It was shown that mononuclear phagocytes (MNP), although present initially in the PBMC cultures, disappeared shortly after the addition of PWM. It was concluded that MNP, although obligatory for stimulation, were no longer required for further transformation of cells after stimulation. At the ultrastructural level stimulation of PBMC with PWM resulted in the generation of 2 types of blast cells. The first type were characterized by the presence of polyribosomes throughout the cytoplasm, large and swollen mitochondria and a few short strands of rough endoplasmic reticulum. The second type of blast cell showed a dispersed pattern of ribosomes, a number of smaller mitochondria and some long strands of rough endoplasmic reticulum. In parallel with a decrease in the number of type I blast cells, plasma cells were found in the stimulated cell cultures. Labeling for surface-membrane immunoglobulins was positive only in type I blast cells and after a longer period in plasma cells. These observations suggest the transformation of type I blast cells into plasma cells, both being derived from the B-cell line. Type II blast cells probably represent transformed cells of the T-cell line.