The mol wt (Mr) of intact murine PRL is approximately 23,000. Immunoblotting analysis shows a 21,000 Mr band in fresh rat and mouse pituitary extracts that cross-reacts strongly with PRL antibodies. The band becomes markedly altered by stimuli known to influence PRL secretion, such as nursing, estradiol benzoate, and perphenazine. By two-dimensional electrophoresis, it splits into at least three components, two of which have more acidic pI than PRL. The tyrosine peptide maps of the three proteins resemble that of PRL. These results indicate that the 21,000 Mr band consists of a cluster of proteins that are structural variants of PRL, raising the possibility that these variants result from alternative splicing of the PRL gene transcript.