IgG antibodies to a 57-kD protein (p57) present in various human and bovine extracts were detected by immunoblotting in the serum of the mother of a baby with congenital heart block, but not in the corresponding cord blood, suggesting specific antibody consumption in the baby. Since this indicates a possible functional role for these antibodies, the antigen target was characterized and the association of the antibodies to heart block was further studied. A human K562 lambda gt11 cDNA library was screened and two clones were identified whose products reacted with the prototype serum. Antibody affinity-purified by use of the cloned gene products reacted on immunoblot with the 57-kD band. Partial sequences of both inserts were identical, but differed from DNA encoding the Ro(SSA) and La(SSB) antigens. Antibodies to the p57 were detected in 10% of systemic lupus erythematosus (SLE) sera, almost exclusively in association with anti-Ro(SSA). Furthermore, they were present in 38% (8/21) mothers of babies with neonatal lupus expressing either cardiac or cutaneous manifestations. Antibodies to this 57-kD protein may be an additional risk factor for neonatal lupus in anti-Ro-positive women. Moreover, disappearance of antibody from cord blood suggests that they may have a role in disease manifestations.