Nineteen percent of the approximately 30,000 members of the Yale community aged 18 through 59 received swine influenza monovalent vaccine (A/New Jersey/1976) during the three days of a mass immunization program in Nov. 1976. Based on 1508 card questionnaires received, 71.2 percent of the vaccine recipients experienced a sore arm, 23.4 percent headache, 13.4 percent chilliness, and 9.7 percent feverishness or fever. The sore arm was judged as severe in 5.9 percent as was the headache in 4.2 percent. Other reactions were regarded as severe in less than 2 percent. All reactions were reported more commonly by women than mean and all decreased with age.Serologic tests carried out at the start of the immunization period revealed that influenza A/New Jersey/1976 antibody was absent from 78.6 percent of the recipients; almost all persons under 25 lacked this antibody. A significant antibody rise occurred in 78.3 percent of those receiving a single dose of monovalent vaccine. Somewhat better antibody responses occurred in 36-59 year olds than in those age 17-25 (84.9 vs 75.5 percent); the geometric mean antibody titer was also much higher (1:136.8 vs 1:31.2). However, the presence of pre-existing homologous antibody did not significantly improve the antibody response to the vaccine. Cross-reacting antibody rises to A/Victoria/1975 were found in 16.2 percent of the recipients of monovalent vaccine.