During a measles epidemic (December 1979-March 1980) in two adjacent villages in Tamil Nadu, 78 of 143 under-fives were affected, eight of whom died, giving an attack rate of 54% and case-fatality rate of 10%. Seven months later 72 children (41 with measles, seven with history of measles in prior epidemics and 24 with no history of measles) were bled to measure measles virus haemagglutination-inhibition antibody. Of the 48 with measles history, 46 had measurable antibody. Surprisingly, of the 24 without a history, 16 had measurable antibody indicating the frequency of subclinical measles during epidemics. The geometric mean antibody titre was lower in infants with measles than in older children (P less than 0.05). The mean titre was lower in those with subclinical measles than in those with clinical measles (P less than 0.01).