Two in vitro-methods for demonstration of cell-mediated immune response are compared: Protein and DNA synthesis for detection of in vitro influenza virus antigen- and mitogen-induced lymphocyte stimulation. Guinea pig spleen lymphocytes sensitized with influenza virus antigen were tested in a microadaptation of the lymphocyte transformation test using 14C- or 3H-leucine and 3H-thymidine. As a positive control for T-cell stimulation PHA-induced lymphocyte stimulation was measured. The following results were obtained: 1. Kinetics of the incorporation of 14C-leucine and 3H-thymidine in lymphocytes incubated with optimal and suboptimal PHA-doses respectively are quantitatively similar but different in time. 2. The results of the protein- and DNA-synthesis stimulation assays were correlated against influenza virus antigens, this could not be described by the comparison of cellular and single humoral parameters. 3. The administration of influenza virus antigens in CFA induced a more intensive cell-mediated reaction than injections of antigens in aqueous suspensions, but the results of both methods of CMI were correlated. 4. The optimal CMI under the experimental conditions described is induced by an administration of 30 to 50 microgram virus protein per animal and by a combined intramuscular--intraperitoneal immunization procedure. 5. The measurement of the early stimulation of protein synthesis in the PSS-test is substantially more rapid than for the classical LTT.