The human blood index, or estimated proportion of the blood meals of a mosquito population obtained from man, is provisionally assessed for certain anophelines from blood-meal samples collected during the period 1959-62 and subjected to precipitin testing at the Lister Institute. In malaria eradication programmes this index is relevant to epidemiological assessment and to the modification of measures to interrupt transmission, since a mosquito's vectorial capacity and the malaria reproduction rate both vary as the square of the human blood index.There are serious difficulties in achieving representative sampling for this index and in interpreting the index obtained. These are discussed in some detail. In practice, the human blood index is often best estimated by applying the unweighted mean of a part-sample collected from human dwellings and one from other types of resting-place.Applying this calculation to the samples under review, it appears that DDT exerts a moderate, and dieldrin a more pronounced, impact on the human blood index of Anopheles gambiae and A. funestus; such an effect, indeed, may be general in house-visiting anophelines. Some 18 anopheline species are tentatively graded as having low, medium or high natural human blood indices. Regular and careful sampling, combined with recording of all relevant information, is recommended in view of the epidemiological and operational importance of the human blood index in assessment of eradication programmes.