Tylophora alkaloids have been shown to have antiasthmatic, antiinflammatory and antianaphylactic properties. Since all these disorders are a consequence of altered immunological status, the effect of these alkaloids on model immune reactions were studied. Crude extract of the leaves of Tylophora indica inhibited delayed hypersensitivity reaction to sheep red blood cells in rats when the alkaloid mixture was administered before and after immunization with these cells. The alkaloid mixture also inhibited contact sensitivity to dinitro-fluorobenzene in mice when given prior to or after contact sensitization. Lymphocytes taken from contact sensitized mice, when treated with tylophora alkaloid in vitro and transferred into naive syngeneic hosts, could suppress the transfer of delayed type hypersensitivity (DTH) response. However, the tylophora alkaloids could not suppress primary humoral (IgM) immune response to SRBC in mice at the same dose. These studies suggest that tylophora alkaloids suppress cellular immune responses when administered at any stage during the immune response.