The prevalence of malaria in children in the two groups was determined through a combination of active and passive surveillance. Any child in the study who presented to one of the dispensaries in the study area or to the national hospital with an illness suggestive of malaria was investigated for possible malaria. If a child had a history of fever or a raised axillary temperature (> 37.5 degrees Celsius) blood films were obtained and the hemoglobin measured. Children found to have malaria were treated with chloroquine which was the first line treatment at the time of the study. In addition to passive surveillance, two cross sectional surveys of all children in the study were undertaken at the beginning and at the end of the malaria transmission season, during which blood films were collected and the hemoglobin measured. Quality control for the reading of blood films was provided by staff of the MRC Laboratories, The Gambia.