Retinol-binding proteins (RBP) are secreted by the porcine uterus under the influence of progesterone and consist of multiple charge forms. Evidence has been previously presented by this laboratory that these uterine RBP are distinct from serum RBP. We have followed the secretion of the uterine RBP during two stages of pseudopregnancy, examined their properties and amino acid sequences, and attempted to clone their cDNA. Analysis of the charge forms present in uterine flushes by anion-exchange chromatography showed that forms 1 (p < 0.01) and 3 (p < 0.05) predominated at Day 13, whereas forms 2 (p < 0.05) and 4 (p < 0.01) were most abundant at Day 45. All four charge forms appeared to form stable complexes with transthyretin (TTR) and were recognized by antiserum to human serum RBP on Western blots. Several cDNA clones isolated from an endometrial cDNA library all appeared to code for a protein identical to classical RBP. Off-blot amino acid sequencing of the first ten residues of two of the more divergent charge forms of uterine RBP indicated complete sequence identity with pig serum RBP. These data suggest that the uterine RBP charge forms may be slightly modified forms of a single protein product corresponding to the classical form of RBP. The change in appearance of the charge forms during pseudopregnancy is probably due to chemical modifications. These modifications do not appear to influence the binding of each charge form to TTR.