Calibrated mixtures of anti-H and anti-L ferritin subunit monoclonal antibodies were used in a sandwich enzyme-immunoassay for the evaluation of all isoferritins. The assay was designed to have overlapping calibration plots for human liver (95% L-chain) and recombinant human H-chain ferritin (100% H-chain). It appeared to recognize all the heart isoferritins including the ones in the middle of the isoferritin spectrum. By direct comparison it was shown that these isoferritins are under-evaluated by the assays for H- and L-subunit-rich ferritins, based on the two separated antibodies. The three assays (for total, H-rich and L-rich ferritins) provide an index of the presence of the isoferritins with intermediate H/L composition. Sera from 30 tumor and non-tumor patients were analyzed. Intermediate isoferritins were found in 2 non-tumor subjects and in none of the 14 patients with Hodgkin's disease or mammary carcinoma. It is concluded that the evaluation of the total and intermediate isoferritins is possible, but does not have an evident clinical significance for tumor monitoring.