Serum ferritin frequently is elevated in patients with neuroblastoma. Isoferritins extracted from neuroblastoma tumors and cells in culture show a wide range from basic (rich in L subunit) to acidic (rich in H subunit) isoferritins. Total ferritin is a combination of basic and acidic isoferritins. Forty-four serum samples from 25 patients with neuroblastoma were measured for basic and acidic isoferritin levels by radioimmunoassay using antibodies to liver (basic) ferritin and HeLa (acidic) ferritin. Normal ranges for basic and acidic serum ferritins were 7 to 142 ng/ml (median, 30 ng/ml) and 0 to 12 ng/ml (median, 3.4), respectively. Basic ferritins in the 44 neuroblastoma sera ranged from 0 to 1460 ng/ml, and acidic ferritins, 0 to 40 ng/ml. Sera with high levels of acidic ferritins always had increased basic ferritins. Thus, acidic/basic ferritin ratios were nearly constant, <0.3 in all sera. There was a significant linear correlation between basic and acidic isoferritins (r = 0.833). These results suggest that neuroblastoma tumors produce both basic and acidic isoferritins and release them into circulation. However, there is no acidic ferritinemia not accompanied by basic ferritinemia. Therefore, the commercial assay for basic isoferritin currently seems sufficient for clinical prognostic purposes.