The individual antibody response to ovalbumin (OA) was studied in groups of hamsters infected with Leishmania donovani. Each group consisted of two infected and two uninfected hamsters. All the animals were immunized to OA at intervals, prior to or following the introduced infection. One week after the initial immunization, each hamster received a second injection making a total of 10 mg OA. This procedure was begun with Group 1, 12 days before infection and was continued in new groups of four hamsters every 3 days until there were 19 groups under study. Each hamster, 24 days after the initial sensitization to OA, was anesthetized, weighed, exsanguinated, and necropsied. Antibody titers to OA measured by tanned cell hemagglutination (HA) from the infected vs. the uninfected hamsters were similar until a moderately severe infection developed (900 Total Spleen Lds"). Above this level of parasitization the anti-OA titers of the infected animals were lower despite increased concentrations of gamma globulin in their serum as shown by electrophoretic studies. The titers of the uninfected animals remained in the same range throughout the experiment. The possible explanations for the depression of the immune response are discussed. © 1969."