[en] Prolactin (PRL) binds to two molecules of PRL receptor (PRLR) through two regions referred to as binding sites 1 and 2. Although binding site 1 has been generally assigned to the pocket delimited by helix 1, helix 4, and the second half of loop 1, the residues involved in receptor binding have not yet all been precisely identified. In an earlier alanine-scanning mutational study, we identified three major binding determinants in loop 1 of human PRL (hPRL) (Goffin, V., Norman, M. & Martial, J. A.(1992) Mol. Endocrinol. 6, 1381-1392). Here we focus on the two other regions that form binding site 1, namely helices 1 and 4. Putative binding residues, selected on the basis of a three-dimensional model of hPRL constructed in this laboratory, were mutated to alanine, and recombinant hPRL mutants produced in Escherichia coli were tested for their ability to bind to the PRLR and to stimulate Nb2 cell proliferation. We thus identified nine single mutations (three in helix 1 and six in helix 4) whose effect was to reduce both binding and mitogenic activity by more than half as compared with wild-type hPRL, indicating the functional involvement of the corresponding residues. Adding these to the three binding determinants identified in loop 1, we now propose a complete picture of PRLR-binding site 1 of hPRL. As we earlier hypothesized, the binding site 1 determinants of hPRL differ from those of human growth hormone, a hPRL homolog.