Decreased sensitivity and incipient resistance of Plasmodium falciparum strains to chloroquine have been reported from Mto-wa-Mbu, in the north-east of the United Republic of Tanzania. In this locality the population had been exposed to chloroquine pressure for about two decades, in the form of medicated salt and through easy availability of the drug itself. In an attempt to find out whether such chemosuppression had influenced the immune response of the population, two seroepidemiological surveys were carried out in March 1981 and March 1982; the second survey was performed to confirm the results obtained in the first one. The humoral immunological response was measured by the immunofluorescent antibody technique. In the absence of information on the immunological profile that existed in the area prior to the introduction of chloroquine in 1960, the results of the present surveys were compared with those obtained in another locality in the north-east of the United Republic of Tanzania in 1967, and in the West Kiang district of Gambia in 1965. The two areas used for comparison exhibited a malaria endemicity similar to that prevailing in Mto-wa-Mbu prior to the introduction of the medicated salt. The results from Mto-wa-Mbu showed a significantly lower proportion of subjects with positive titres and a lower geometric mean titre in all age groups.A reduction in the humoral immunological response might be explained by the drug pressure that has been exerted in the area for many years. The depressed immune response found at Mto-wa-Mbu, however, was so marked that other factors may have contributed to its establishment.In view of the importance of these findings, it is recommended that further, longitudinal serological studies be conducted in the field to assess the effects of chemosuppression on the immune response of the protected populations.