Human placental plasma membrane vesicles were cultured for up to 5 days in the presence of spleen cells from (BALB/c X C57BL/6By) F1 hybrid mice. The membrane preparations either inhibited the uptake of [3H]-thymidine ([3H]-TdR) by destroying the viability of the T-cell population or stimulated weak lymphocyte division which was primarily an expansion of the T-cell population. This differential effect was dependent on the membrane concentration in culture and the length of time the membrane preparation had been stored. Membrane preparations that inhibited [3H]-TdR uptake could be converted into stimulatory-type membranes by preincubating them at 37 degrees for several days. This conversion coincided with a change in status concerning membrane susceptibility to disruption by cytotoxic non-T-cells present in the spleen of unimmunized animals. The conversion from stimulating-type membrane into inhibitory-type was never observed. Throughout these cultures the generation of cell-mediated cytolysis could not be detected.