To evaluate the relationship between tumor burden and circulating immune complexes (IC) in children with neuroblastoma (NBL), we studied sera collected at intervals from patients with disseminated (Stage III or IV) NBL. Sera from 10 of 12 patients contained IC by the Raji cell assay at some time during the first 9 to 11 months of the study. Higher IC levels were observed in sera of female patients. Fluid-phase C1q binding tests detected IC in only 16% of sera. IC measurements by either assay did not correlate with tumor burden. However, serum IC levels, as measured by the Raji cell assay, correlated significantly with serum antibody to bovine serum albumin (BSA) (rs = 0.54; p less than 0.001, rs = r as determined by Spearman rank correlation test). Measurement of anti-BSA antibodies in sera from the 12 patients, tested serially for circulating IC, and from five additional patients revealed that these had significantly higher anti-BSA activity than was found in sera from 13 age-matched controls. Sera from females also had relatively high levels of anti-BSA. Levels of antibody to bovine gamma-globulin and casein were not abnormal. Three sera with high IC levels (greater than 800 micrograms equivalents of heat-aggregated IgG) and relatively low anti-BSA activity appeared to contain "hidden" antibodies to BSA. These were demonstrated by measuring the increase in the ability of sera to bind 125I-BSA after they had been briefly acidified and then neutralized in the presence of the labeled BSA. The possible relevance of these results to the pathophysiology of NBL is discussed in light of earlier work that reported that serum IC levels correlate with the stage of the disease in NBL.