We show that a brief exposure of human peripheral blood mononuclear cells (PBMC) to adenosine or to theophylline results in a mitomycin C resistant regulatory activity. Adenosine induced suppression is also detectable in a lymphocyte subpopulation (T4+ enriched, originally described as helper inducer) resistant to the theophylline induced loss of capacity to form spontaneous rosettes with sheep erythrocytes (TTR). This activity is apparently dependent on the production of a soluble factor(s) since supernatants from adenosine treated TTR (SnA) exert a significant inhibition on the proliferative response of resting lymphocytes. On the contrary SnA increases the concanavalin A (ConA) preactivated lymphocytes proliferation. Similar results are detectable on the proliferative response in the mixed lymphocyte reaction (MLR). Perhaps these effects are related to different Interleukin 2 (Il 2) receptor expression on the cell surface of the resting and preactivated populations. A slow moving band corresponding to a protein of Mr of 64,500 and isoelectric point 7.6 is present in SnA. Only a slight Il 2 activity is detectable either in SnA and in control supernatant (SnC). These findings suggest that SnA may be a dynamic regulator of the early stages of lymphocyte activation.