Various plant lectins differentiate in their ability to induce early (chemiluminescence and thermostable rosette formation) and later (DNA synthesis) activation events of human mononuclear cells. Both phytohaemagglutinin and concanavalin A induced a cell activation detectable by the three investigated parameters. Wheat germ agglutinin was characterized as an efficient stimulator in both assays of early events whereas the induction of DNA synthesis measured by [3H]-thymidine incorporation was low. In contrast, pokeweek mitogen did evoke only a weak chemiluminescence signal, but induced a small number of thermostable rosettes and a high [3H]-thymidine incorporation. The results obtained from experiments with mononuclear cells of twelve healthy donors did not show a firm correlation between the investigated three parameters of cell activation.