Using immunocytochemistry with the sensitive avidin-biotin complex method and an antibody to the β sub-unit of rat luteinizing hormone (rat LHβ) provided by the National Hormone and Pituitary Program, NIADDK, to identify gonadotrops, the presence of angiotensin II-like immunoreactivity in the gonadotrops of the anterior pituitary gland of male rats was confirmed. However, using an antibody to rat prolactin, no angiotensin Il-like immunoreactivity could be found in lactotrops. This finding is at variance with a previous report from this laboratory, in which the lactotrops were identified with an NIADDK antibody to human prolactin. The possibility that the antibodies to human prolactin cross-reacted with rat LH was therefore investigated by a noncompetitive radioimmunological binding assay. At the same dilution used in immunocytochemistry, two different NIADDK antihuman prolactin antisera displayed 49% and 24% binding of labeled rat LHβ. Similar cross reactions were demonstrated immunocytochemically. There was no cross reactivity between rat LHβ and the antibody to rat prolactin. Consequently, the staining reported to be in lactotrops in the previous study was probably in gonadotr