Cynomolgus macaque monkeys (Macaca fascicularis) were immunized twice intramuscularly, either with a conventional non-adjuvanted subunit vaccine or with a candidate immune-stimulating complex (iscom) vaccine, each containing 10 micrograms envelope glycoprotein of a recent human influenza A(H3N2) virus (A/Netherlands/18/94). In contrast to the macaques vaccinated with the classical subunit vaccine, those immunized with the iscom vaccine developed high titres of specific IgM, IgA and IgG serum antibodies, as well as high titres of haemagglutination-inhibiting and virus-neutralizing serum antibodies. Also, specific proliferative T cell responses were only found in the iscom-vaccinated monkeys and their levels were similar to those found in monkeys experimentally infected with the homologous virus. Upon intratracheal challenge with the homologous virus, the iscom-vaccinated monkeys were completely protected from detectable virus replication in lungs, pharynx and nose, whereas those vaccinated with the classical subunit vaccines were not, or were only partially protected. The kinetics of specific serum antibody development in the iscom-vaccinated monkeys after challenge were quite similar to those of monkeys after secondary infection with the same virus. In contrast, the post-challenge kinetics of serum antibody development in the monkeys vaccinated with the classical subunit vaccines resembled those of naive monkeys, confirming that these vaccines only provided limited protection in such animals.