Prolactin has been separated from growth hormone (GH) in extracts from sheep and cattle pituitaries, but not when human pituitaries were used. The prolactin activity found in most human growth hormone (HGH) preparations (Li & Liu, 1964; Forsyth, Folley & Chadwick, 1965) supports the hypothesis that HGH and human prolactin (H-Pr) are biologically and immunologically related and suggests that both activities reside in the same molecule (Irie & Barrett, 1962). However, Chen & Wilhelmi (1964) have reported that prolactin activity can be separated from that of GH by chromatography. Moreover, Pasteels, Brauman & Brauman (1963), using tissue cultures of human pituitary cells, found a rapid decrease of GH activity in the medium but an increase in prolactin activity. The present report describes the immunological properties of a human prolactin preparation (Apostolakis, 1965) more potent than any previous preparation. The activities of batches XVI-R8 and XVII-R8 of human prolactin