We previously showed that 71-kDa heat shock cognate protein (HSC70) functions as a cellular receptor for gp46 protein via the gp46–197 region, corresponding to Asp197 to Leu216 of human T-cell lymphotropic virus type 1 (HTLV-1), leading to cell-to-cell transmission of HTLV-1. We found that HSC70 protein was contained in goat serum and casein used as blocking agents in the usual ELISA method. Here, it was demonstrated that HSC70 contamination in the blocking agents causes a false-negative result in the detection of anti-gp46–197 antibody in serum samples from HTLV-1-infected individuals. By using ELISA without the blocking agents, we detected antibodies recognizing the HSC70-binding site of gp46, and the anti-gp46–197 antibody specifically appeared in sera from patients with HTLV-1-associated diseases. The frequency of serum anti-gp46–197 antibody-positive individuals was 98% and 100% among ATLL and HAM/TSP patients, respectively, but only 6% among asymptomatic HTLV-1-infected carriers (ACs). The antibody titer in ATLL and HAM/TSP patients was higher than that in ACs (P>0.002 for ATLL; P>0.0001 for HAM/ TSP). These findings suggest that appearance of the anti-gp46–197 antibody is a predictive marker for the onset of HTLV-1-associated disease.