Spleen cell cultures of radiation chimeras (thymectomized, lethally irradiated mice repopulated with bone marrow cells and thymocytes bearing different chromosomal markers) were stimulated by phytohemagglutinin (PHA) and F1 allogeneic spleen cells. Karyotypic analyses showed a marked predominance of T mitoses on the 2nd and 3rd days of culture followed by a strong predominance of B mitoses on the 4th and 5th days. Analysis of cells undergoing their first mitoses showed that the majority of T mitoses on day 3 resulted from continuous T cell division, and that most cells entering their first mitoses at that time were of B type. Mixed lymphocyte cultures (MLC) of chimeras immunized against allogeneic spleen cells showed sometimes, but not always, a response different from "primary" MLC, with an earlier and stronger predominance of BM mitoses. The role of stimulated T cells in the induction of B mitoses was shown by (a) the incapacity of T-depleted spleen cells to be stimulated by PHA or in primary or secondary MLC, and (b) the restoration of the mitotic response of B cells to PHA by adding to the T cell-depleted culture either a very small number of T cell (identified by their different karyotype: "in vitro chimeras") or the cell-free supernatant of a 24 hr MLC.

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