A PHASE of immunodepression is characteristic of experimental malaria infections and, in mice, immune responses to antigens as diverse as sheep red blood cells1, tetanus toxoid2, oncogenic viruses3 and Toxoplasma gondii4 are diminished. From immunological and epidemiological viewpoints it is important to know what happens when animals are infected with another agent during this period of immunodepression. The agent must be carefully chosen; it must be able and likely to coexist with the malaria parasite in vivo, be easy to monitor and elicit a well defined immune response. Trypanosoma musculi, an avirulent murine trypanosome, fulfils these criteria. There is no cross immunity between it and rodent malarias5, the immune response has been very carefully characterised6–8 and the progress of the infection can be monitored easily by taking minute quantities of blood. We have found that T. musculi infections are considerably enhanced in mice coincidentally infected with an avirulent malaria parasite, Plasmodium yoelii (P. berghei yoelii)9.