Repression of colony formation reversed by antiserum to mouse thymocytes.

Marrow cells derived from C57BL/6 mice form many fewer splenic colonies in irradiated C57BL/6 x C3H F1 hybrid recipients than in irradiated C57BL/6 recipients (repression of colony formation). This effect is reversed by treatment of the hybrid recipients with active antiserum to mouse thymocytes. The repression phenomenon cannot readily be explained in immunological terms; hence the effect of the antilymphocyte serum on this phenomenon may not result from immunosuppression in the usual sense.