The activity of various most widely used "polyclonal" mitogens, such as PHA, Con A and PWM on human peripheral blood lymphocyte subpopulations, after separation into the two main functional classes (B and T), was investigated. For separating the lymphocyte subclasses, the E-rosette formation technique has been followed, using the gradient centrifuged pellet and supernatant respectively. For the mitogens' activity evaluation, micro-culture system has been performed and 3H thymidine uptake has been assessed. The PHA has been shown to be much more active on the T lymphocyte subclass, though the mitogen cannot be kept very selective. Very much higher activity on the T subclass shows the Con A, which could be considered selective, if we hypothesize an at least partial receptor competition at lymphocyte membrane level between the mitogen and sheep erythrocytes. The PWM has been found very little active and not specific for either lymphocyte subclass.