Rabbits were immunized with diphtheria toxoid combined with complete Freund's adjuvant. Half of the animals were started on intramuscular injections of chloramphenicol 24 hr before the injection of the antigens. There was a general depression of protein synthesis in the immune system in the presence of chloramphenicol, but a greater effect on the synthesis of antibody than on the synthesis of proteins necessary for reproduction and maturation. In contrast to the finding of antibody in cells of the spleen and in the circulation of the control animals, those animals receiving chloramphenicol did not have measurable circulating antibody, and their spleens contained only a few cells with intracytoplasmic antibody late in the course of the experiment. Cytologically there was maturation of potential antibody-producing cells in the red pulp and nonfollicular white pulp of the spleen while the animals were receiving chloramphenicol. These cells developed more slowly, and were fewer and smaller than those of the control animals. They had numerous small, electron-opaque particles in their cytoplasm early in development. Ribosomes were synthesized, though fewer in number. The endoplasmic reticulum formed more slowly.

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