Phosphorylation of chick progesterone receptor (PR) was attempted by incubating tissue minces from estrogen-primed oviducts with ortho [32P]phosphate in the absence and presence of different steroids. The phosphorylated PR was immunopurified from the cytosol using anti-PR monoclonal antibody alpha PR22 (Sullivan et al., 1986). Although all three known peptides of PR, peptides B (110K), A (79K) and the 90 kDa nonhormone binding peptide (heat shock protein, hsp-90), were phosphorylated, the presence of only progesterone increased the degree of phosphorylation of receptor peptides A and B and the dissociation of the hsp-90 from the PR heterooligomer. Other steroids, cortisol, estradiol and dihydrotestosterone (DHT) had no effect on the phosphorylation or on the dissociation of hsp-90 from the PR. Incubation of phosphorylated PR at 23 degrees C or at 4 degrees C with 0.3 M KCl or 10 mM ATP also caused dissociation of the hsp-90. Presence of progesterone in vitro increased dissociation of the hsp-90 and the subsequent PR binding to DNA-cellulose. Transformation in vivo or under cell free conditions did not alter the degree of phosphorylation of PR peptides A and B. Our results demonstrate that PR is phosphorylated in a hormone-specific manner and that its transformation by various agents leads to loss of the hsp-90 from the oligomeric structure without an apparent involvement of dephosphorylation.