The Aedes egypti mosquito fed consistently on guinea-pigs in a 2-hour period (n = 61), the mean percent feeding rate (+/- S.D.) being 88.84 +/- 9.32. Of a total of 34 different compounds systematally administered in guinea-pigs and tested for their effect on the mosquito biting rate using the above model, five: heparin, sodium fluoride, aminocaproic acid, thiourea and dithiocarb partially reduced the biting rate. The results are consistent with the view that certain aminoacids or proteins of blood or tissues serve as 'pheromones' attracting the mosquito to guinea-pigs.