Serum and nasopharyngeal IgA antibody levels were estimated in 20 malnourished children and 20 matched healthy controls after immunization with a single dose of live attenuated measles or poliovirus vaccine. Seroconversion and serum neutralizing antibody titres were comparable in the two groups. Secretory IgA antibody was detected significantly less often in undernourished children; the time of its first appearance was delayed-and its maximum level was significantly lower. Impaired secretory antibody response in malnourished children may contribute to slow inadequate recovery from viral and enterobacterial infections and predispose to lifethreatening complications.