Upon incubation in rabbit reticulocyte lysate, the unactivated progesterone receptor (PR) associates with the heat shock proteins hsp90 and hsp70, the immunophilins FKBP52, FKBP54, and CyP-40, and another protein p23. We have previously described a protein complex between p23, hsp90, and the immunophilins that forms in rabbit reticulocyte lysate in the absence of the PR. Immunodepletion of p23 from lysate prevented the binding of hsp90 and CyP-40 to the PR, suggesting that hsp90, CyP-40, and p23 bind the receptor as a complex. We have further examined the properties of this p23 complex to determine how it is involved in receptor assembly in vitro. Use was made of three chemical probes, sodium molybdate, the nonhydrolyzable ATP analog, 5'-adenylylimidodiphosphate, and the hsp90-binding agent geldanamycin. Molybdate has previously been shown to stabilize the heat-induced dissociation of hsp90 and p23 from the PR. This stabilization is not mediated through the PR, as molybdate stabilizes the heat-induced dissociation of hsp90 from p23 even in the absence of the PR. Molybdate also stabilizes both p23 and PR complexes under conditions of low ATP and magnesium concentration. The ATP analog, 5'-adenylylimidodiphosphate, which does not support the assembly of PR complexes, promotes both the assembly and stabilization of p23 complexes. Geldanamycin disrupts p23 complexes, and when PR complexes are treated with this agent, p23, CyP-40, and some hsp90 are lost from the receptor. Thus, all three of these chemical agents appear to target the p23 complex, which is thought to enter at the last step in the assembly of the PR complex. A model is presented to relate these findings to previous models and another complex between hsp90, hsp70, and p60 that appears to be an intermediate in PR assembly.