The effect of Leishmania glycoconjugate in the vector was investigated using Phlebotomus papatasi artificially infected with a Leishmania major strain that this vector does not transmit in nature. Glycoconjugate of the vector-specific strain of L. major was added to the infective meals of some fly groups and the success of infections with or without this substance was compared 4 d later. In the absence of glycoconjugate the parasites survived in 15.6% of the flies, while the addition of 0.5 mg/ml and 2 mg/ml raised the rate of infection to 34.3% and 63.0% respectively. Undigested blood was found in the stomach of 7.8% of the infected flies, whereas following meals with parasites and 2 mg/ml glycoconjugate it was present in 37% of the flies. The results demonstrated that this glycoconjugate increased the viability of the parasites in the unsuitable vector and delayed digestion of the infective meals. In a parallel experiment the glycoconjugate of L. donovani did not affect the survival of the parasites.