Serum samples from different groups of adults were tested for HBsAg and IHxAg, using a complement-fixation microtest and the Indian-ink immune reaction, respectively. (i) In healthy men 18-24 years of age, living in camps in closed communities, HBsAg was demonstrated in 1.5%, IHxAg in 12.2%, and both antigens in 0.7%. The incidence of HBsAg positivity seems to be age-dependent and influenced by environmental factors. (ii) For patients hospitalized with liver and/or biliary-tract diseases other than hepatitis, the respective percentages were 10, 13.5 and 4.5%. (iii) Of the cases clinically diagnosed as infectious hepatitis (IH, hepatitis A) or serum hepatitis (SH, hepatitis B), 14% were positive for both antigens whereas 10% were double-negative; 76% were positive for either HBsAg or IHxAg. In two-thirds of the single-positive cases the demonstrated antigen agreed with the clinical diagnosis, in one-third the unexpected antigen was present. (iv) SGPT and thymol turbidity values agreed better with the serological findings that with the clinical diagnosis. The number of days in hospital appeared to be related to both the serological findings and the clinical diagnosis. The clinical course was the most severe for those having both antigens in blood. (v) IHxantibodies from early convalescence were sensitive, those from a later stage were resistant, to 2-mercaptoethanol. (vi) No correlation was found between the presence of IHxAg and that of the rheumatoid factor. (vii) The IHx Indian-ink reaction is disturbed by the presence of labile serum proteins while the essentially similar reverse passive haemagglutination reaction was not affected by them. (viii) Testing for IHxAg seems to be a procedure valuable in the differential diagnosis of IH and SH, though the results are less convincing in adult age than in childhood.