A human T-leukaemic cell line, HSB.2-C5B2, which produces high levels of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) when stimulated with phytohaemagglutinin (PHA) plus IL-1, was recloned to obtain spontaneous variants in IL-2 production in response to the stimuli. In these subclones, the ability of one clone to produce IL-2 correlated well with that to produce IFN-gamma. Three C5B2 subclones: clone no. 28, a high IL-2 producer, clone no. 61, an intermediate IL-2 producer, and clone no. 40, a non-producer, were selected and examined for differences in signal transduction mechanisms. Since the three subclones were shown to express about the same number of IL-1 binding sites with similar affinities, the loss of ability to produce IL-2 was not due to decreased cell-surface receptor or changes in receptor property. In support of this, IL-1 induced expression of the IL-2 receptor (Tac/p55 antigen) to the same extent on the three subclones. The levels of conventional intracellular second messengers were compared and it was revealed that loss of responsiveness was closely related to the subclones' degree of (poly)phosphoinositide (PI) turnover, protein kinase C (PKC) activation and cyclic AMP formation in response to PHA. Moreover, resting intracellular cyclic AMP concentrations were found to be increased in subclones with attenuated IL-2 production. These results indicate that the variation of IL-1-induced production of IL-2 and IFN-gamma in this T-cell line is attributed to the difference in the PHA-mediated signal transduction pathway and, presumably, to the different regulation of intracellular cyclic AMP.