Light microscopical studies on the uptake of blood-borne horseradish peroxidase (HRP) in large areas of the testis have been scarce because of the difficulty of staining HRP in testes with well-preserved morphology. However, observation of exogenous HRP in all areas of the testis enables detection of regional tissue injury induced by toxic chemicals or immunization. In the present study, the localization of blood-borne HRP in the murine testis was investigated light microscopically using plastic-embedded testes and post-embedding histochemical methods. Mice were injected intravenously with HRP, and then perfused with 2.5% glutaraldehyde and 3% paraformaldehyde in 0.1 M phosphate buffer. The fixed testes were immediately removed, dehydrated, and then embedded in plastic without cutting them into small pieces. The prepared sections treated by the diaminobenzidine method exhibited intense HRP activity with well-preserved testis morphology. It was noted that many interstitial macrophages had endocytosed HRP. In particular, HRP-endocytosing macrophages were concentrated around the tubuli recti. The testicular capsule, containing many lymphatic capillaries and vessels, was also loaded with HRP. In the subcapsular interstitium, free HRP in the lymph space accumulated, but the staining intensity was weak compared to that in testicular macrophages. No HRP infiltration into the lumen of the seminiferous tubules was observed at the light microscope level; however, HRP staining was detected in tubular walls and epithelial cells lining the rete testis and tubuli recti, indicating that these regions are permeable to HRP.