Constitutive lymphokine production by cells isolated from mice injected with keyhole limpet haemocyanin (KLH) or from mice undergoing an acute graft vs host reaction (GVHR) was very low, but could be markedly increased by T cell receptor (TCR) ligation. This suggested that in vivo levels of lymphokine production are much lower than those induced by in vitro stimulation. Serum lymphokine titres were consistent with this possibility, and analysis of lymphokine mRNA levels using S1-nuclease protection demonstrated that in vitro-stimulated cells from normal, KLH and GVHR mice all had markedly increased levels of lymphokine transcripts relative to levels found in vivo. A novel method combining limiting dilution analysis with polymerase chain reaction amplification of cDNA was developed that showed that these differences in levels of lymphokine production were due at least in part to differences in the frequencies of lymphokine mRNA-containing cells. Studies of the means by which differential lymphokine production is achieved demonstrated that CD4+, CD8+ and cytotoxic T lymphocyte (CTL) clones all express a common, restricted set of lymphokines in response to a defined in vitro stimulus, but that individual in vivo primed cells can express at least seven distinct patterns of lymphokine production.