A new radioimmunoassay (RIA) procedure using a double-sandwich technique was developed for the detection of hepatitis A antigen (HAV) in crude faecal extracts for patients involved in three outbreaks of type A hepatitis. Stools were obtained from 24 residents suffering from acute hepatitis A and from six children who remained asymptomatic throughout the epidemic. In addition, the HAV detection was performed in sera from 13 patients with hepatitis. HAV was detected in stools as early as five days before and as late as five days after the onset of jaundice. In this procedure, positive activity was only found in stools from patients with type A hepatitis, but not in negative controls. HAV was not detected in acute-phase sera. The double-sandwich RIA test used appears to be a reliable test for the large-scale screening of HAV in stool samples from patients suffering from type A hepatitis.