GATA-3 and T-box expressed in T cells (T-bet) play central roles in Th-cell development and function. Consistently, studies in mice document their selective expression in Th1 and Th2 cells, respectively. In contrast, it is not clear whether these genes are regulated in human Th cells. Here we show that T-bet expression is polarized to a comparable degree in human and mouse Th-cell cultures, while only mouse GATA3 is subject to substantial regulation. This did not reflect differential skewing efficiency in human versus mouse cultures, as these contained similar frequencies of IFN-gamma- and IL-4-producing cells. However, GATA-3 was expressed at significantly higher levels in human IL-4-producing cells enriched via capture with monoclonal antibodies (mAbs) against the PGD(2) receptor, CRTH2, the best selective Th2-cell surface marker to date. Along with increased IL-4 and GATA-3, CRTH2(+) Th cells isolated from Th2-skewed cultures or the circulating memory pool exhibited markedly decreased IFN-gamma and T-bet expression. Thus, the human GATA-3 gene is not regulated in response to polarizing signals that are sufficient to direct Th2-specific expression in mouse cells. This postulates the involvement of an additional level of complexity in the regulation of human GATA-3 expression and stresses the existence of nontrivial differences in the regulation of human versus mouse T-cell function.