We have recently described a bone marrow culture system which is able to maintain a portion of the precursor-T (pre-T) cell compartment of adult murine marrow in vitro, in the presence of interleukin-3 (IL-3), for at least 2 weeks. However, because growth in IL-3 might also induce the differentiation of the pre-T cells, it is necessary to determine the extent to which the developmental potential of the pre-T cells is altered during their residency in vitro. Previously, we analysed the progeny of cultured pre-T cells and compared their intrathymic development, their appearance in the periphery, and their V beta gene utilization to that of the progeny of fresh pre-T cells. Within these parameters, the cells derived from cultured marrow cells did not differ significantly from cells derived from fresh marrow cells. However, these studies did not allow us to determine the functional status of the T-cell progeny of cultured marrow. In the work presented here, we analysed the functional potential of T cells which were derived either from fresh pre-T cells or pre-T cells which had been maintained for 1 week in vitro. The T-cell mediated functions analysed included mitogen- and alloantigen-induced proliferation, IL-2 production, and generation of cytotoxic T cells. We found that the cultured pre-T cells were capable of giving rise to mature, immunocompetent T cells which did not differ significantly from the progeny of fresh pre-T cells in their functional potential.