Pokeweed mitogen (PWM)-stimulated cells show maximum transformation after 72 hr in culture. When optimal concentrations of PWM are used, the transformed cells do not exceed 50 to 60% of the total. After 72 hr the population of cells includes small and medium-size lymphocytes, large phytohemagglutinin (PHA)-like blast cells and a unique intermediate sized cell type. The intermediate sized PWM cell is characterized by an eccentric nucleus with clumped heterochromatin, a well developed Golgi apparatus, ribosomal aggregates and a well developed rough-surfaced endoplasmic reticulum. The fine structural features differ importantly from those consistently seen in PHA transformed cells. The possible immunologic significance of PWM stimulation of human peripheral lymphocytes in vitro has been considered.