An isoimmune chimpanzee serum was found to contain, in addition to expected antibodies of the simian-type specificity anti-Lc, a fraction of anti-Rho specificity, as proved by parallel tests with standard human anti-Rho sera as well as by absorption experiments using human and chimpanzee red cells. On the other hand, the two human anti-Rho sera tested in parallel were found to contain not only antibodies of specificity anti-Rho but also antibodies capable of detecting Lc specificity on the chimpanzee red cells. The reciprocal relationship between human and chimpanzee isoimmune sera constitutes the first example of symmetrical cross-reactivity between closely related species. The new source of anti-Rho reagents is expected to contribute additional information on the human Rh-Hr blood group system.