CERTAIN viral infections affect the immune response of the host to heterogenous antigens. Murine leukaemia viruses, such as Moloney, Friend, Rauscher and Gross passage A virus, inhibit antibody formation1-4. In addition, the passage A virus depresses cellular immunity5. On the other hand, lactic dehydrogenase virus (LDV) enhances humoral antibody formation in mice and prevents the development of immunological tolerance to human gamma globulin6,7. The work described here was undertaken to determine the effect of LDV on two cellular immune reactions - skin allograft rejection and the graft-versus-host reaction8. © 1969 Nature Publishing Group.