The success of malaria eradication campaigns depends on the use of all methods which make for a better understanding of the biology and behaviour of mosquito vectors. One such method is precipitin testing, by which it is possible to identify the human or animal origin of blood meals of mosquitos and thereby to determine their host preferences and vectorial importance, both generally and locally.In 1955, the World Health Organization in agreement with the Lister Institute of Preventive Medicine, Elstree, England, set up a precipitin test service related to entomological surveys in malaria eradication programmes and available to national research and WHO field personnel. The purpose was to stimulate interest in the study of bionomics of Anopheles species, to facilitate the identification of blood meals of Anopheles, to eliminate experimental errors by the use of a standardized technique and highly sensitive antisera, and finally to apply the results in the strategy of malaria eradication.The results obtained over the past five years are summarized in tabular form. The study-the largest ever undertaken-included 51 species of Anopheles and 56 377 tests, of which 93.9% yielded positive results, are reviewed. The available knowledge of the vectorial importance of 39 species of Anopheles is compared with their human blood ratio, this term being used to express the percentage of human blood in relation to all precipitin tests found positive.