Activation of human peripheral blood monocytes by pokeweed mitogen (PWM) results in the expression of interleukin 2 receptor(IL-2R) p55 chains, which are absent on resting monocytes. By dual-fluorescence flow cytometry, we found PWM induced detectable numbers of IL-2R+ cell which were further identified as LeuM3+ monocytes (22.91% of the LeuM3+ cells were IL-2R+ within 24h, and 32.17% in 48h). In addition, LPS can also induce IL-2R on 15.39% of LeuM3+ cells. On the contrary, other mitogens such as PHA or Con A, and cytokines as IFN-gamma, IL-2, M-CSF, TNF-alpha and IL-1 beta showed no influence on the IL-2R expression on monocytes. It was also noticed that PWM itself had no direct effect on HLA-DR antigen expression on LeuM3+ cells. The addition of IL-2 to PWM-pretreated IL-2R positive monocytes significantly augmented their tumoricidal activity. Thus monocytes when activated underwent a series of phenotypic and functional changes including the expression of IL-2R which may provide an important immunoregulatory pathway.