Recombinant, fully bioactive, authentic human prolactin (aut-hPRL) has been synthesized in transformed Escherichia coli HB2151 bacteria in a soluble, non-glycosylated form, which is secreted into the bacterial periplasm. Use was made of a bacterial expression vector, containing tac promoter-controlled sequences for the translation enhancer from bacteriophage T7 gene 10, and for a cellulase leader peptide from Cellulomonas fimi joined to sequences coding for aut-hPRL. This vector was derived from a previously described vector containing sequences of an hPRL variant, tag-hPRL (containing a 12-amino-acid peptide tag at the N-terminal end), using site-specific mutagenesis to delete the tag sequence. SDS/PAGE, partial N-terminal amino acid sequence analysis, Western blot analysis and Nb2 lymphoma cell in vitro bioassay indicated correct processing of the hormone. Periplasmic secretion of aut-hPRL, as measured by immunoassay, was relatively low (approx. 0.08 microgram/ml per A600 unit), in contrast to that of tag-hPRL which was approximately 8-fold higher, apparently a consequence of the tag sequence. This is the first report describing periplasmic secretion of biologically active, authentic hPRL.