We have analyzed heat shock proteins (hsp) and immunophilins in the cytosol of squirrel monkey and human cells (Abs 2). These are proteins which are thought to form a heterocomplex with glucocorticoid receptors and tailor them into a high-affinity state. Twenty ?g of cytosolic protein from human and squirrel monkey B-lymphoblasts were electrophoresed on 8% denaturing gels and transferred to nitrocellulose. Proteins of interest were detected by Western blot analysis and visualized by the ECL chemiluminescence system. Densitometric scanning of the immunoreactive bands showed that the levels of hsp90, hsp70, p60, p48, and p23 were similar between the two species. However, proteins of the immunophilin family showed quite dramatic species-specific differences in their abundance and apparent molecular weight. FKBP52 (hsp56), a 59 kDa FK506 binding protein, was 3-fold lower in squirrel monkey cytosol, whereas cyclophilin 40 (Cyp40), a 40 kDa cyclosporin-binding protein, was 1.7-fold lower. On the other hand, FKBP51 (54 kDa in human, 52 kDa in squirrel monkey) was 7.5-fold higher in squirrel monkey cytosol than in human cytosol. Therefore, immunophilin proteins in squirrel monkey cells are expressed at different stoichiometric ratios than in human cells. As these immunophilins are hsp90-binding proteins and are thought to compete for the same sites on hsp90, their differential expression in squirrel monkey cells may alter glucocorticoid receptor-heterocomplex assembly. Future studies will focus on the role of these immunophilins in the decreased glucocorticoid sensitivity of this primate.