We analysed the Giemsa stained blood smears by microscopy to quantify parasitaemia as the proportion of infected red blood cells (%). This was determined by counting infected erythrocytes in 100 randomly selected microscopic fields under 1000 magnification and by relating the number of infected to the total number of erythrocytes per microscopic field, determined from five pictures taken every 20 fields (mean number of erythrocytes per microscopic field: 306 ± 69 (s.d.), n = 550).