Abstract: Since many chemicals including biochemical inhibitors and natural materials are difficult to get in large amount, a more sensitive method for preliminary screening is needed.The authors found that an intraperitoneal injection of Na125I mixed with an emulsifier composed cf lanolin and liquid paraffin could keep higher concentration for 3-4 days in mice, while Na125I solution alone could only persist for one day. On this basis, the solution or fine suspension of drugs (e.g. chloroquine and pyrimethamine) in water was mixed with the emulsifier and injected once intraperitoneally into mice three hours after Plasmodium yoelii infection. Blood smears were taken on the fifth day, the number of infected RBC were counted and all the procedures were just the same as the four days oral administration method recommended by WHO for another group of infected mice.By comparing the two methods mentioned above, when one day's dosage of oral administration method was used for injection, very similar results were obtained. Thus, the new method seems to be more labour-saving and especially suitable for the screening of drugs which were limited in supply.