An antibody, which is distinct from the HBsAg, and reacts with antigenic sites on Dane particles HBcAg and HBeAg, was studied by radioimmunoprecipitation of radioactive intact hepatitis B virions in sera obtained early in the course of acute hepatitis type B. The antibody, previously termed anti-Dane particle (anti-DP) antibody, was reactive with Dane particles and HBsAg particles obtained from HBeAg-positive sera but not with HBsAg particles from anti-HBe containing sera. The expression on virus particles of the evoking antigen correlated with levels of binding sites for polymerized human serum albumin (pHSA) as detected by solid-phase radioimmunoassay. In acute hepatitis B sera, levels of anti-DP antibody activity showed inverse correlation with expression of pHSA receptors on circulating virus particles, although the two reactivities were not mutually exclusive. In inhibition experiments, pHSA blocked precipitation of Dane particles by anti-DP positive sera, while native human albumin and polymerized bovine albumin had no effect. The inhibition by pHSA of the anti-DP reaction appeared specific since identical concentrations of pHSA did not interfere with precipitation of virus particles by anti-HBs. Affinity chromatography studies with anti-DP insolubilized on Sepharose 4B columns showed selective binding to the gel of radioactive Dane particles; 125I-HBsAg was not reactive. The binding of Dane particles to anti-DP columns was completely inhibited when virus particles were applied to the gel in pHSA; pretreat-ment of the column with pHSA did not affect the reaction. These results and the demonstration that anti-DP positive sera were free of anti-pHSA antibodies indicate that anti-DP reacts with antigenic determinants on Dane particles that are involved in or by pHSA binding sites. Anti-DP positive sera and IgG fractions inhibited expression of pHSA receptors on purified virus particles. Our findings support suggestions that the anti-DP response may be relevant to the phase of virus neutralization, on the basis of the hypothesis that the virus receptor for pHSA could play a role in mediating virus access to hepatocytes which may have similar albumin receptors on their membrane.