We have investigated the correlation between results obtained by three different methods (semi-quantitative RT-PCR, ELISA and ELISPOT) used to measure cytokine expression by mouse leukocytes. The production of the cytokines tumour necrosis factor-alpha (TNF alpha), interferon-gamma (IFN gamma) and interleukin-4 (IL-4), was analysed with all three methods. In a simple experimental murine in vivo model of leukocyte stimulation, consisting of a single intravenous injection of anti-CD3 antibodies followed by a short incubation in vitro, the results obtained with spleen cells for each of the three cytokines differed greatly, depending on the method used. For TNF alpha, a significant increase in RNA was observed upon stimulation, whereas the number of spot-forming cells did not increase and the protein was not detectable in serum or in cell culture supernatants by ELISA. In vitro cultured splenocytes showed a strong correlation between all three methods for IFN gamma. Upon stimulation, the amount of RNA for IL-4 increased in parallel with the secretion of the cytokine and the number of spot-forming cells. However, high numbers of spot forming cells were observed in controls. We conclude that, depending on the specific aim of an investigation, combinations of different methods have to be chosen carefully in order to detect activation of leukocytes for cytokine expression.