In 1973, Feinstone et al. (1), with the technique of immune electron microscopy (IEM) 2 (2), detected virus particles in acute-phase stool suspensions obtained from patients with type A hepatitis. This technique was useful in further characterizing hepatitis A virus, but required the expertise of a skilled electron microscopist. The need for a rapid, more sensitive technique that could qualitatively evaluate large numbers of specimens became apparent. Recent success in our laboratory with a microtiter solid phase immunoradiometric assay (micro-SPIRA) for detecting hepatitis B core antigen in the sera of patients with post-transfusion hepatitis B suggested that a similar application to the hepatitis A problem might be appropriate (3). The method involves the coupling of unlabeled antibody to an insoluble matrix which can then extract immunologically reactive antigen from the test specimen. Detection of the specific antigen is measured after interaction with a second radiolabeled antibody.