SummaryThe antibody response to sheep red blood cells (SRBC) has been measured in C57B1 mice infected with the intra‐erythrocytic piroplasm Babesia microti. The primary antibody response is severely reduced or abolished when antigen is administered at the time of maximum parasitaemia. The secondary antibody response of infected mice, which had been primed with SRBC, is reduced but retains the characteristics of a secondary response. Mice injected with SRBC at maximum parasitaemia failed to become primed to that antigen: these mice gave a primary antibody response to a second injection of SRBC given after the infection had become sub‐patent. B. microti, therefore, can completely prevent the induction of memory, but only partially inhibit the expression of mem