SUMMARYThe in vitro action of the immunosuppressive agent Cyclos-pirin A has been investigated using porcine cells. Lymphocyte proliferation induced in response to transplantation antigens and phytohaemagglutinin (PHA) was inhibited by this agent at doses that failed to inhibit mitogenisis in response to sheep anti-pig IgM, growth of kidney cell monolayers, and leukocyte migration.