In vitro addition of an autoclaved noninfective measles virus preparation to cultured lymphocytes markedly inhibited the cellular proliferation induced by phytohemagglutinin or tuberculin, as measured by lymphoblastic transformation, and incorporation of tritiated thymidine and uridine. This suppression was greater than that exerted by live virus and was dose-dependent. No gross evidence of cytotoxicity was observed, as measured by changes in cell number or trypan blue exclusion.