The proliferative effects of the mitogens phytohaemagglutinin (PHA), concanavalin A (Con A), pokeweed mitogen (PWM), and staphylococcal protein A (SpA) were investigated using two different methods which enable immunological marker analysis of proliferating cells: either surface marker labelling followed by BrdU incorporation or screening of metaphases after surface marker labelling. Therefore peripheral blood mononuclear cells from six healthy volunteers were stimulated with these four mitogens. Both PHA and Con A gave rise to more CD8+ than CD4+ proliferating cells. PHA, but not Con A, induced B-cell proliferation as well. PWM mainly caused T-cell proliferation. SpA also appeared to be a potent T-cell mitogen in addition to its capacity to induce B-cell proliferation. However, in contrast to the other mitogens SpA predominantly stimulated CD4+ cells.