On Western blot analysis, serum IgG from a healthy human subject reacted with a stress-induced protein, having an apparent molecular mass of 70 kDa, from PC12 cells. 2. This serum IgG also reacted with hsp70 (70-kDa heat-shock protein) purified from the bovine brain. 3. In 5 out of 34 healthy human subjects, IgG antibodies against hsp70 were detected. 4. These antibodies were directed against the stress-inducible 72-kDa protein, but did not cross-react with the constitutive 73-kDa protein.