Antisera to bovine serum albumin (BSA) react with biosynthetic products of the LAN-1 neuroblastoma cell line. This immunological reaction was shown by analysis of immunoprecipitates prepared with anti-BSA and products of this cell line intrinsically labeled by the incorporation of 3H- or 35S-labeled amino acids. Moreover, antibodies in sera from two patients with neuroblastoma precipitate intrinsically labeled macromolecules produced by these tumor cells. Release of antigens cross-reactive with BSA by neuroblastomas may explain, in part, the high levels of antibody to BSA and circulating immune complexes containing hidden or "blocked" antibodies to BSA found in some patients with this tumor.