The expression of human cyclophilin 40 (CyP-40) as a glutathione S-transferase fusion protein has provided a means to identify cellular components that are in association with this ubiquitous protein. When the fusion protein was coupled to a GSH affinity matrix, heat-shock protein 90 (hsp90) was found to be the predominant associated protein in all tissue extracts examined. The relatively high concentration of each of these proteins in various tissues indicates that the dimeric complex exists in concentrations that exceed those of the inactive steroid receptors of which each protein is a component. Association does not occur with heat-shock protein 70 and is not affected by cyclosporin A (CsA). Independent expression of two domains of CyP-40 permitted dissociation of N-terminal isomerase and CsA binding activity from the hsp90 binding site, which is located at the FKBP-59-like C-terminal region. The biological association of CyP-40 with hsp90 in many tissues may reflect a conjoint role in protein folding and trafficking.