We studied 375 chronic hemodialysis patients for evidence of hepatitis B virus infection using first- and second-generation monoclonal radioimmunoassays. These assays employ high-affinity monoclonal antibodies produced against antigenic determinants that reside on hepatitis B surface antigen. Such assays have a lower limit of detection for hepatitis B surface antigen-associated determinants in serum of approximately 55 and 15 pg/ml, respectively. We found that 14 of 375 chronic hemodialysis patients were positive for hepatitis B surface antigen by both polyclonal and monoclonal radioimmunoassay. However, an additional 17, some of whom had chronic hepatitis and hepatocellular carcinoma, were identified as harboring hepatitis B virus infection only by the monoclonal radioimmunoassays. Thus the monoclonal radioimmunoassays improved the hepatitis B virus detection rate by 120% (3.7% vs. 8.3%). More importantly, 6 of the 17 monoclonal radioimmunoassay-reactive patients had no serologic evidence of recent or past hepatitis B virus exposure as shown by the absence of antibodies to the hepatitis B core and surface antigens in the blood. We conclude that there are hemodialysis patients with hepatitis B virus infection undetectable by conventional polyclonal radioimmunoassays.