The decision to implement a mass immunization program with A/New Jersey/8/76 (Hsw1N1) influenza vaccine provided a unique opportunity to evaluate immunological responses during pregnancy. Fifty-nine pregnant and 27 nonpregnant women participated in this study. Influenza virus hemagglutination-inhhibition antibody titers were determined to A/New Jersey/8/76 (Hsw1N1), A/Japan/305/57 (H2N2), and A/Hong Kong/8/68 (H3N2) before and after a single dose of monovalent (200 chick cell agglutination units) influenza A/New Jersey/8/76 (Hsw1N1) vaccine. The difference in titers between pregnant and nonpregnant women was insignificant. Treatment of the sera with 2-mercaptoethanol disclosed a similar immunoglobulin M response to the vaccine in both groups. The mean fold rise in heterologous antibody titer was similar in pregnant and nonpregnant women. This study demonstrated that pregnant women were able to respond to an original myxovirus antigen, influenza A/New Jersey/8/76, in a manner equivalent to nonpregnant, age-matched controls.