A neutralization enzyme immunoassay (N-EIA) was used to determine the neutralizing serum antibody titers to influenza A/Taiwan/1/86 (H1N1) and Beijing/353/89 (H3N2) viruses after vaccination of 51 human immunodeficiency virus (HIV) type 1-infected individuals and 10 healthy noninfected controls against influenza virus infection. Overall, the N-EIA titers correlated well with the hemagglutination-inhibition (HAI) titers that were observed in the same samples in a previous study (F. P. Kroon, J. T. van Dissel, J. C. de Jong, and R. van Furth, AIDS 8:469-476,1994). The N-EIA appeared to be more sensitive than the HAI test. Significantly more fourfold or higher rises in N-EIA titer and higher mean N-EIA titers occurred in HIV-infected individuals with > or =200 CD4+ cells per microl than in those with <200 CD4+ cells per microl.