Schistosoma mansonieggs induce a rapid and pronounced Thresponse which, based on cytokine secretion patterns, at day 3 post priming is Th0)‐like and at day 10 is Th2‐like. To establish whether or not the day‐3 cells have been programmedin vivoto develop into Th2 cells, they were cultured for 7 days to becomein vitroequivalents of day‐10in vivocells. Following this culture period, the population was approximately 75% CD4+, 22% CD8+, 6% B220+and capable of producing IL‐2, IFN‐γ, IL‐4, ‐5 and ‐10 upon stimulation. This Th0‐like status was confirmed by the observations that in response to mitogen IL‐4 and IFN‐γ production are both CD4+‐cell dependent and that IFN‐γ and IL‐4 are produced concomitantly by single cells. These data suggest that ThO cells persistin vivo, but are incapable of secreting IFN‐γ at day 10 due loan inhibitory factor which does not develop or is labilein vitro. This concept is supported by ihc surprising observation that day‐10 LN cells, which are Th2‐like immediatelyex‐vivo, rapidly gain the ability to secrete