Incubation of intact reticulocytes with chloramphenicol at concentrations over 10(-3) M caused inhibition of protein synthesis and conversion of polyribosomes to single ribosomes. At 3 x 10(-3) M-chloramphenicol these effects were completely reversible, but reversibility decreased as the chloramphenicol concentration was raised above this level. 3 x 10(-3) M-chloramphenicol also caused a 15% decrease in the ATP content of reticulocytes. Evidence was obtained that the intracellular concentration of chloramphenicol in reticulocytes was approximately equal to the concentration in the incubation medium. L(+)threo chloramphenicol inhibited protein synthesis in reticulocytes, and chloramphenicol inhibited the synthesis of both hemoglobin and RNA in bone marrow cells. These results indicate that the action of chloramphenicol in these cells is different from that in bacteria.