The aim of the study was to determine if autoantibodies have distinctive qualitative or quantitative characteristics in children with congenital heart rhythm disorders compared to unaffected infants. Sera of fifty-three infants with different heart rhythm disorders and eight unaffected children born to SLE mothers were tested for antibodies to Ro(SS-A), La(SS-B) and U1RNP (including recombinant A-, C-, 68 kD-proteins) using enzyme immunoassays and immunoblot. 21 sera from affected and 8 sera from healthy children reacted in counter-immunoelectrophoresis with Ro(SS-A). 8/8 infants with third degree atrioventricular (AV)-block, 4/8 with first degree AV-block, one with sinusbradyarrhythmia (SB) and another with SB/first degree AV-block were anti-52 kD Ro(SS-A) positive compared to 7/8 healthy infants. Infants with third degree AV-block had significantly higher anti-52 kD Ro(SS-A) levels than those with the milder heart rhythm disorders (P < 0.01) and the healthy group (P < 0.002). Anti-La(SS-B) antibodies were detected significantly (P < 0.01) and in significantly higher titres (P < 0.01) in AV III than in other types of congenital heart block. Anti-U1RNP-68 kD antibodies were positive in 3/8 healthy, 2/8 first degree AV-block and in 1/8 third degree AV-block cases. Third degree AV-block was more frequently found in female infants (P < 0.05). It was concluded that the influence of antibody levels to the 52 kD Ro(SS-A) and La(SS-B) components and the association of anti-52 kD Ro(SS-A), La(SS-B) and -60 kD Ro(SS-A) antibodies, as well as the sex, on the severity of congenital AV-conduction defects suggests involvement of these factors in the pathogenesis of these disorders.