THE murine t-complex encodes t-complex polypeptide-1 (TCP1)¹, which is constitutively expressed in almost all cells, and upregulated during spermatogenesis^2,3. Mammalian sequences have >96% identity with each other, and >60% identity with Drosophila melanogaster and yeast orthologues^4–9. TCP1 is essential in yeast⁹, and is postulated to be the cytosolic mammalian equivalent of groEL^10,11. We report here that, in the native state, murine and human TCP1 is distributed throughout the cytosol as an 800K–950K hetero-oligomeric particle in association with four to six unidentified proteins and two Hsp70 heat-shock proteins. Negative-stain electron microscopy indicates that the structure is two stacked rings, 12–16 nm in diameter. Therefore, despite similarities with the chaperonin 60 proteins, these data indicate that TCP1 is biochemically and structurally unique. We suggest that TCP1 may represent one of a family of molecules in the eukaryotic cytosol involved in protein folding and regulated in part by their heteromeric associations.