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The success of malaria eradication campaigns depends on the use of all methods that 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 available to national research institutions and field staff of malaria eradication projects. The results of the tests carried out in 1959-64 are now presented in summary form; the data were obtained from nearly 41 000 blood smears collected from 79 species of Anopheles. In addition, the previously published results of the 1955-59 period are retabulated and data are presented on nearly 27 000 tests carried out independently at the National Institute of Communicable Diseases, Delhi, India, on Anopheles from Ceylon, India and Nepal. Altogether the review covers some 124 000 precipitin tests on 92 Anopheles species; about 93% of the tests gave a positive result with one or other of the antisera used, but attention is chiefly paid to the proportion of blood-meals taken on man.There are practical difficulties in achieving representative sampling of Anopheles populations for determination of the human blood index, but some can be overcome by increased care in sampling from a representative selection of biotopes. In areas that have been sprayed with insecticide, an attempt should be made to include mosquitos knocked down by the insecticide after feeding.