Caryotypic analysis of the cells dividing in mouse parent-hybrid MLC showed an F1 hybrid cell proliferation, which varied depending upon the source of lymphoid cells used: strong in spleen MLC (sometimes equal to that of the parental cells), less marked in lymph node cell MLC, and most often absent in MLC between cortisone-resistant (CR) thymocytes. MLC between parental spleen cells and F1 CR thymocytes showed, however, that in certain conditions of culture F thymocytes can also proliferate. Using parental or F1 spleen cells lacking T lymphocytes, it was found that F1 cell proliferation is entirely dependent upon the presence of parental T cells, but does not require the presence of T lymphocytes among the F1 cells. Immunofluorescence analysis of the blasts observed in one-way MLC showed that about 70% of the parental blasts were T blasts, and 25%B blasts (containing a high proportion of plasmablasts); among the F1 blasts, there was also the same percentage of B blasts and plasmablasts, but many of the T blasts bore only small amounts of T-cell antigen (MTLA), and there was also about 20%of unstained blasts, possibly T blasts bearing MTLA in amounts undetectable by immunofluorescence. The possibility is discussed that the F1 responding T cells belong to a subpopulation performing a suppressive function; MLC lacking F1 T cells showed increased [3H] thymidine incorporation. The proliferation and differentiation of parental and F1 B cells may result mainly from an unspecific, "polyclonal" triggering.