A finger prick-swab method of blood specimen collection was qualitatively and quantitatively compared with the conventional venipuncture method for HBsAg and anti-HBs determinations by radioimmunoassay (RIA). The new method consisted of pricking the finger, collecting 0.1-0.2 ml of blood with a cotton-wool swab, and eluting the swab in 1 ml of 1% bovine albumin in saline containing 0.1% sodium azide. Using chimpanzees seropositive for HBsAg or anti-HBs, comparisons were made of RIA results of: (a) whole blood, haemolysed blood, serum, and plasma; (b) paired finger prick samples and serum; (c) dilutions of finger prick samples and serum; and (d) different volumes of blood on swabs. Field studies were carried out at two institutions where hepatitis B was hyperendemic to compare results from paired finger prick and serum specimens assayed by the RIA and haemagglutination techniques. The laboratory studies showed that swab RIA values for anti-HBs were significantly lower than serum values and that for HBsAg, swab values were significantly higher than serum values. In HBsAg tests, the field studies showed 100% agreement between the two methods; in anti-HBs tests, the finger prick method showed 85% agreement with positive sera. Because of the logistics of collecting and processing blood serum, the finger prick-swab technique may be a valuable aid in large-scale seroepidemiological surveys for hepatitis B.