1. A sandwich-type enzyme-linked immunosorbent assay (ELISA) is described for quantitation of secretory IgA (sIgA) in human serum, as well as an ELISA and a radioimmunoassay (RIA) for measurement of secretory component (SC) in human serum. Samples were reduced and alkylated prior to the measurement of SC. 2. Healthy individuals (N = 53) presented low levels of SC (median, 0.9 mg/l). The protein levels were significantly elevated when compared with the controls, in sera of women during the second (N = 31; median, 1.5 mg/l) and third (N = 35; median, 4.2 mg/l) trimesters of pregnancy and in sera of patients with alcoholic cirrhosis (N = 38; median, 2.4 mg/l) and acute viral hepatitis (N = 25; median, 2.4 mg/l). SC levels of women in the first trimester of pregnancy (N = 24; median, 0.5 mg/l) did not differ from the controls. 3. sIgA levels were also significantly elevated when sera of women in the third trimester of pregnancy (N = 41; median, 25.4 mg/l) and sera of patients with alcoholic cirrhosis (N = 32; median, 75.0 mg/l) or acute viral hepatitis (N = 38; median, 28.5 mg/l) were compared with controls (N = 49; median, 9.0 mg/l). Women in the first (N = 25; median, 7.7 mg/l) and second (N = 29; median, 10.2 mg/l) trimester of pregnancy did not present levels statistically different from the controls. 4. The results obtained for SC by RIA and ELISA were positively correlated (rs = 0.88; P less than 0.001). sIgA levels determined by ELISA were also positively correlated with the results of RIA-SC (rs = 0.77; P less than 0.001) or ELISA-SC (rs = 0.79; P less than 0.001). 5. The assays described are specific, relatively simple to perform, and can be useful for the study of the secretory immune system.