The effect of progesterone (P) on the cytokine production profile of Ag-specific human CD4+ T cell lines and clones was investigated. T cell lines specific for purified protein derivative or streptokinase (SK) derived in the presence of P exhibited significant increased ability to produce IL-5 in comparison with T cell lines derived in the absence of P. Moreover, IL-4 was significantly increased in SK-specific T cell lines derived in the presence of P in comparison with SK-specific T cell lines derived in the absence of this hormone. In addition, SK-specific T cell lines generated in the presence of P developed into T cell clones showing a Th0-, instead of Th1-like, cytokine profile. Furthermore, SK-specific T cell clones with an established Th1 profile of cytokine secretion did express mRNA for, and produced detectable amounts of, IL-4 when stimulated with P in combination with insoluble anti-CD3 mAb. Combined stimulation with P and insoluble anti-CD3 mAb also enabled Th1 clones to express CD30 on their surface membrane. These results indicate that P can favor the development of Th cells producing Th2-type cytokines and is an inducer of both transient IL-4 production and CD30 expression in established Th1 cells. Thus, P production at the placental level may be responsible, at least in part, for increased production of Th2-type cytokines which have been implied in fetal allograft survival and maintenance of successful pregnancy.