Reactivity and immunogenicity of three inactivated, zonally purified, monovalent influenza A/swine virus vaccines were studied in children and adults. Each dose of vaccine contained either 400 chick cell-agglutinating (CCA) units/0.5 ml or 200 CCA units/0.25 ml. The vaccines contained either whole virus or ether-extracted, subunit virus with or without 1.5 mg of A1PO4/0.5 ml. Children younger than 10 years of age received a half dose. Substantial system reactions, including temperature increases of 2.2 F-4.9 F, were observed in all children who received whole-virus vaccines. In contrast, ether-extracted, subunit vaccines (with or without A1PO4) were minimally pyrogenic in 185 subjects. Two doses of subunit vaccine in subjects younger than 25 years of age were immunologically equivalent to a single dose in older subjects. We concluded that two doses of ether-extracted, subunit virus vaccine with Hsw1N1 antigen, administered at least four weeks apart, are serologically effective for immunization of seronegative subjects of any age and that this dosage regimen should be used in young children in whom whole-virus vaccines are unacceptably reactive.