In higher eukaryotic cells, the Hsp90 chaperone protein is found in complexes with other proteins, in addition to the substrate protein. These other proteins include Hsp70, p60, and several peptidyl-prolyl cis-trans isomerases (immunophilins). We utilized affinity chromatography to investigate whether Hsp82, the Hsp90 of Saccharomyces cerevisiae, is found in similar complexes in that organism. Six histidine residues were fused to the N terminus of Hsp82 to yield a fusion protein (Hsp82FP) with affinity for a nickel-ion matrix. Hsp82FP was shown to have wild-type function. In addition, when mammalian substrates of Hsp90 (glucocorticoid receptor and p60v-src) were expressed in yeast cells, these proteins bound to the affinity matrix only when isolated from cells containing Hsp82FP. Yeast homologs of Hsp70 (in particular, members of the Ssa subfamily), p60 (Sti1), and a 45-kDa immunophilin homolog were also isolated in this manner, by virtue of their specific, stable association with Hsp82. Thus, Hsp90 functions as part of a highly conserved macromolecular complex in eukaryotes.