An enzyme-linked immunosorbent assay (ELISA) was developed which permits the assay of specific secretory immunoglobulin A (sIgA) antibodies against hepatitis B surface antigen (HBsAg) in saliva. The assay is based on the binding of sIgA antibodies present in saliva to microtitre plates coated with excess of F(ab')2 anti-secretory component antibodies, followed by the addition of specific antigen, HBsAg and finally peroxidase-labelled anti-HBsAg. The assay is fast, simple, reproducible and antigen specific as shown by total absence of inhibition of specific antigen by unrelated antigens but significant inhibition of labelled anti-HBsAg by unlabelled anti-HBsAg. The values obtained for hospital personnel exposed to hepatitis infections (0.068 +/- 0.083 U/ml) and for post-icteric hepatitis B patients (0.062 +/- 0.033 U/ml) were significantly higher than values in control subjects (0.013 +/- 0.006 U/ml).