Incorporation of tritiated [3H]thymidine byChlamydia psittaci6BC was achieved by growing the parasites in chick embryo yolk sac explants which were exposed to exogenous labeled thymidine. These labeled, purified chlamydiae were next observed by autoradiography within mouse peritoneal macrophages. The number of silver grains remained constant in the cytoplasm of macrophages throughout the developmental cycle of the parasite. The proliferation of labeled chlamydiae in macrophages was confirmed by Giemsa staining and immunofluorescence. Chlamydiae have never been successfully labeled with thymidine in earlier studies when assayed in cultured mammalian fibroblasts. It is suggested that a critical factor in the successful incorporation of thymidine in chlamydiae may be the host–parasite system used.