The hepatitis B virus infects only humans and higher apes. Viruses similar to the human hepatitis B virus (hepadna viruses) have been discovered in several nonprimate species including woodchucks, ground squirrels, and domesticated ducks. To search for other models of hepatitis B virus infection, we screened serum specimens from 64 exotic animals (24 species), 56 domesticated animals (6 species), and 52 laboratory animals (3 species). Samples were tested for deoxyribonucleic acid polymerase by enzymatic assay and for hepatitis B surface antigen and antibody and antibody to hepatitis B core antigen by radioimmunoassays. All sera were negative for deoxyribonucleic acid polymerase, hepatitis B surface antigen, and antibody to hepatis B core antigen suggesting that none of these animals harbored hepadna viruses in serum. However, 48% of the sera from 58% of the 33 species were reactive for antibody to hepatitis B surface antigen. This reactivity was blocked by human serum positive for hepatitis B surface antigen but not by control human serum. The antibody to hepatitis B surface antigen was generally present in low titer (95% were less than or equal to 1:16) and was often directed against subdeterminants of hepatitis B surface antigen (anti-d, anti-y, or anti-w). Characterization of the antibody to hepatitis B surface antigen by gel chromatography, sucrose density ultracentrifugation, affinity chromatography, and chemical inactivation suggested that it was entirely or predominantly immunoglobulin M antibody. Thus, many animals species have naturally occurring immunoglobulin M antibody to hepatitis B surface antigen detectable by radioimmunoassay. This antibody could arise as a result of either the intermittent spontaneous maturation of clones of antibody to hepatitis B surface antigen forming lymphocytes or exposure to environmental antigens that share epitopes with hepatitis B surface antigen. Similar naturally occurring antibody to hepatitis B surface antigen may be present in some humans.