ABSTRACT Complement fixation (CF) was compared to hemagglutination inhibition (HI) as a method for identifying antibody responses to influenza virus vaccination. CF assays were performed at two different laboratories using paired (pre- and postvaccination) sera from 38 vaccinated laboratory employees; HI assays were performed at a third laboratory. As expected, most vaccinees (31/38 = 82%) responded to at least one of three influenza virus antigens as measured by HI. In contrast, only 21% (8/38) of vaccinees showed a response by CF at laboratory 1, and only 29% (11/38) showed a response by CF at laboratory 2. These findings indicate that due to low sensitivity, CF assays should not be used to assess the antibody response to influenza virus vaccination.