In a two-center, comparative trial, 344 adults were randomly assigned to receive a single dose of inactivated split-virion (Imovax Gripe) or sub-unit (Agrippal S1) influenza vaccine (1999-2000 formulations). For analysis, study groups were subdivided into adult (18-60 years old) and elderly (over 60 years) subjects. Blood was drawn immediately before and one month after vaccination, safety was evaluated using a blind-observer design based on reporting of solicited and unsolicited adverse events. Both vaccines were very well tolerated, had similar reactogenicity profiles, and elicited fewer reports of reactions in elderly individuals. Post-vaccination Imovax Gripe induced seroprotective antibody titers against the three vaccine strains in 94-99% of adults and 88-97% of elderly subjects, compared with 88-100% and 88-98%, respectively, of those given Agrippal S1. In conclusion, the split-virion and sub-unit influenza vaccines had similar safety and reactogenicity profiles, and elicited satisfactory immunity in adult and elderly subjects. However, higher post-vaccination geometric mean titer (GMT) values in response to the B strain were seen with the split vaccine Imovax Gripe, giving it a better overall immunogenicity.