GH4C1 cells increase storage of rat PRL and accumulation of dense core secretory granules when they are treated with 300 nM insulin, 1 nM estradiol, and 10 nM epidermal growth factor. In the experiments reported here, intracellular rat PRL increased from 4% of the total amount produced in 24 h in control cultures to 15% in treated cultures. When GH4C1 cells were transfected with DNA sequences so that they coexpressed human PRL, the storage of rat PRL was no longer induced by hormone treatment; transfected clones contained less than 5% of the total produced in 24 h in both control and treated cultures. The sum of rat and human PRL produced by these clones was not more than rat PRL produced by the untransfected cells, so the storage capacity of the cells was not exceeded. The transfected clones made between 1 to 40 times more rat than human PRL. Release of human and rat PRL was stimulated by depolarizing the cells, indicating both still were in a regulated pathway, even though storage could not be induced. Mutations of human PRL with threonine substituted for asn31 or alanine substituted for ser34 did not block induction of rat PRL storage when coexpressed in GH4C1 clones. We conclude the ability to increase rat PRL storage is a process with marked specificity because it is inhibited by relatively low amounts of human PRL and inhibition requires asn31 and ser34 in human PRL. Such specificity is consistent with a receptor-mediated mechanism that concentrates PRL into dense cores of secretory granules.