A malaria prophylaxis programme for 100 000 children in one rural district of Tanzania was carried out under the very favourable conditions of 1) a government genuinely committed to rural public health, 2) a well organized system of village government for distribution of the drug to children, and 3) free chloroquine supplied without interruption by WHO to the project area. The project failed to suppress malaria for a combination of the following reasons. Small delays in distribution because of poor communication, vehicle breakdowns, bad roads, key people being away or too busy resulted in the drug not being available for ingestion at regular intervals. A few children from families with marginal social status were excluded by local leaders. Some people were not convinced that regular chloroquine taking might prevent children's malaria and therefore saved it to treat fever in all family members. Various survey techniques were used to discover why children might refuse to swallow tablets, and as many as 28% of children complained of vomiting, as many as 56% complained of itching, and other unfavourable qualities of chloroquine were indicated.