A heterologous immunoassay for 2-hydroxyoestrogens has been established in which antibodies raised against 2-hydroxyoestradiol-17-succinyl-BSA serve as binding protein and 2-hydroxyoestrone-17-cmo-[125I]iodohistamine as radioligand. Lipophilic serum components competing for binding sites in this system were defined as total 2-hydroxyoestrogens'. The underlying assumption of specificity was supported by the pattern of cross-reactivity evaluated with structural related steroids and o-diphenols and by the fact, that an additional chromatography of the serum extracts preceding the competing reaction had little if any effect. Sensitivity: 2.8 +/- 1 pg/tube; accuracy: Y = 0.91x + 2.2; r = 0.989; precision: 5.8% intra-assay; 6.5% inter-assay. The following concentrations (+/- standard deviation) were found in the sera of healthy subjects. Young men: 29 +/- 5 pg/ml (n = 11); women follicular phase: 32 +/- 8 pg/ml (n = 25); luteal phase: 53 +/- 13 pg/ml (n = 23); postmenopausal women: 13 +/- 4 pg/ml (n = 10); pregnant women 11th--20th week: 70 +/- 16 mg/ml (n = 64); 36th--40th week: 240 +/- 23 pg/ml (n = 40); newborn cord blood: 604 +/- 43 pg/ml (n = 48).