The safety and immunogenicity of a single dose of virosome-formulated hepatitis A vaccine was evaluated in healthy seronegative Thai volunteers. Immunization elicited primarily mild transient reactions which did not interfere with normal activities. All subjects possessed > 20 mIU of anti-hepatitis A virus antibody per ml of serum one month after immunization. Such a prolonged rise in antibody titre is characteristic of virosome-formulated vaccines. Protective titres (> 20 mIU ml-1) were maintained by all subjects over the one-year observation period.