Primary in vitro proliferative responses of naive T cells to antigens other than superantigens and alloantigens have been little studied. Two tissue culture techniques have been reported which support in vitro antigen priming of T cells. These methods require various degrees of cellular manipulation and culture vessels other than standard microtitre plates. We report here that primary proliferative responses to non-recall antigens can be readily obtained using unselected human PBMC prepared from either adult or cord blood. Cells proliferate whether cultured in 2 ml volumes, 200 microliters microcultures or 20 microliters hanging drops. The variation in the proliferative responses increases as the culture volume is decreased such that considerable errors are apparent when Terasaki culture plates are used. The lowest stimulation indices are also observed in the 20 microliters microcultures. Nevertheless, similar response patterns are noted for the differing culture vessels; generally, proliferative responses reach peak magnitude only after 7 days of culture. The initial concentration of PBMC in culture influences the magnitude of the reactions such that halving the cell numbers frequently leads to greater than 50% reduction in the measured responses. The results of this study indicate that neither a specialised culture vessel nor complex cellular manipulation are required for in vitro priming of T cell immunity. Consequently, this area of immunology should be readily amenable to further study.