The measurement of intracellular ATP levels using luciferinluciferase induced bioluminescence was used for the evaluation of proliferative responses of human mononuclear cells to lectins. A linear relationship was observed between cell numbers and ATP contents in samples, and high reproducibility was confirmed. Lectin-stimulated mononuclear cells increased ATP contents with the passage of time, and a significant difference from non-stimulated cells was obtained 48hrs after culture. Furthermore an interesting results that the pre-treatment with mitomycin C strikingly increased the ATP level of lectinstimulated cells, but not that of lectin-non-stimulated cells was obtained. These findings suggest that proliferative response of mononuclear cells can be measurable without the use of isotope, and that more early event occurring in stimulated cells may be analyzed by this simple method.