Two interferon (IFN) messengers were synthesized in phytohemagglutinin (PHA)-activated lymphocytes: IFN-gamma mRNA and a messenger hybridizing with the IFN-beta 2 probe. They were induced rapidly and declined at a stage when overall RNA was still efficiently transcribed. The IFN-beta 2 mRNA (15S) present in the lymphocytes was slightly different from its fibroblastic counterpart (14S). The kinetics of the accumulation and decay of both lymphocyte IFN messengers differed when assessed by hybridization with the two IFN probes, IFN-gamma mRNA was not detected before mitogenic activation and accumulated for up to 15 h postactivation, while IFN-beta 2 mRNA accumulated even in the absence of PHA activation for up to 5 h, even though the activation raised the IFN-beta 2 mRNA level at 5 h. The disappearance of IFN messengers was prevented when cycloheximide was added 5 h after PHA activation, when the transcription of both messengers had already been turned on, suggesting the presence of the repressor mechanism proposed for IFN-beta 1 and IFN-beta 2 mRNAs in fibroblasts. In the absence of PHA activation, cycloheximide did not induce IFN-beta 2 mRNA transcription as it did in fibroblasts and moreover prevented the accumulation of the messenger observed in the control cells. In contrast to IFN-beta 2 mRNA, cycloheximide treatment of lymphocytes produced a slight accumulation of IFN-gamma mRNA. This accumulation was already detectable 6 h posttreatment and its level remained unchanged for up to 24 h. Addition of actinomycin D, 5 h after PHA activation, did not impair the shut off and accelerated the decay of IFN messengers.