Protein synthesis in clear cells of mouse epididymis was investigated by quantitative autoradiography after intravenous injection of tritium-labeled leucine, arginine, and lysine. Labeling occurred in the cytoplasm after 2 min and persisted for 240 min after injection. The silver grains were located not only on the organelles involved in protein synthesis (rough endoplamsic reticulum, Golgi apparatus) but also on those implicated in reabsorption (multivesicular bodies). Quantitative analysis showed a peak in radioactivity in the rough endoplasmic reticulum, at 10-15 min, followed by a peak in the multivesicular bodies at 15 min. Labeling of the Golgi apparatus reached its maximum at 30 min. Radioactivity in the apical plasma membrane remained low and relatively stable. These results indicate that clear cells synthesize proteins, a large part of which is transferred to the multivesicular bodies that participate in the cellular absorptive function.