The constitutive HSP70 purified from CHO cells, which indicated a single band in SDS-polyacrylamide gel electrophoresis, showed multiple bands in native-polyacrylamide gel electrophoresis. These results indicate that the protein may exist in oligomeric forms. After crosslinking the oligomers with glutaraldehyde, SDS-polyacrylamide gel electrophoresis showed three protein bands of molecular weight 70 kDa, 153 kDa, and 200 kDa corresponded to monomer, dimer, and trimer, respectively. The relative amount of oligomeric forms was dependent upon ATP concentrations: it increased upon hydrolysis of ATP or decreased upon incubation with high concentrations of ATP (1-10 mM). Autoradiographic analysis of the native polyacrylamide gel electrophoresis of HSP70 following incubation with [gamma-32P]ATP revealed that ATP bound to only monomer. These results suggest that the equilibrium between oligomeric forms is dependent on ATP concentrations. Nonetheless, during heat shock, both monomer and oligomer might be indistinguishably associated with some proteins, probably denatured proteins.