A rapid and precise method for the assay of cell-mediated immune response basing on protein synthesis stimulation of mitogen-activated guinea pig lymphocytes is modified in a way that enables the study of virus-immunological problems. When used as a micromethod it has the following advantages over conventional methods: short-term cell culture, need of low quantities of cells and rapid preparation of great numbers of samples for radioactivity measurements. In this study we report the results of comparative experiments on measuring lymphocyte stimulation after addition of PHA and stimulation of sensitized lymphocytes following contact with homologous influenza virus antigen in vitro. The most important reaction parameters are as follows: 5-6 . 10(5) spleen lymphocytes/microculture in microtiter plates, use of Eagles's MEM cell culture medium without leucine, supplemented with HEPES buffer and 10% autologous guinea pig serum; optimum lymphocyte stimulation by addition of 0.5 microliter PHA or 0.1-1.0 microgram virus protein/ml; immuno-stimulation by PHA can be measured in vitro already after 6 h and by influenzavirus antigen already after 24 h.