Immune adherence hemagglutination (IAHA) was compared to complement fixation (CF), using standard procedures, for serological testing of human sera with a number of commercially available antigens. The antigens included herpes simplex, measles, cytomegalo-, and influenza (type B) viruses, as well as Mycoplasma pneumoniae and Chlamydia psittaci (Chlamydia group). The IAHA test was found to be as specific as the CF test, but 4 to 20 times as sensitive with all antigens tested. Antigen titers were also higher with the IAHA method, and the time required to complete the test was only 4 h for the IAHA method, compared with 20 h for the CF method. The increased sensitivity of the IAHA test should permit its use for determination of immunity, as well as for serodiagnosis of recent infections.