We report that an aqueous solution containing 10(-3) M adenosine triphosphate (ATP) and citrate-phosphate-dextrose-adenine (CPDA-1) can effectively replace transfusable human red blood cells in an in vitro Klun and Debboun bioassay system for evaluating chemicals for mosquito feeding-deterrent activity, using either Aedes aegypti or Anopheles stephensi. These species fed with similar avidity through collagen membrane covering aqueous 10(-3) M ATP plus CPDA-1 or red blood cells in CPDA-1 supplemented with ATP. In a 2nd experiment, we evaluated the feeding-deterrent activity of N,N-diethyl-3-methylbenzamide and a newly discovered natural product chemical, (-)-isolongifolenone, against these 2 mosquito species. We found that the feeding-deterrent efficacy of the 2 chemicals was similar whether the feeding stimulant was red blood cells supplemented with ATP or ATP alone with CPDA-1. Since the use of human red blood cells in bioassays raises important health and logistic issues, aqueous ATP with CPDA-1 is a reasonable alternative to human blood cells for routine in vitro chemical screening.