The various Leishmania species are flagellated protozoans, responsible for a wide spectrum of human diseases. The sequence of the L. major genome is nearing completion and a large proportion of the identified genes have yet to be ascribed functions. DNA microarrays containing PCR-amplified DNA from a random amplified genomic library of L. major Friedlin (LmjF) [Mol. Biochem. Parasitol. 113 (2001) 337] were hybridized with fluorescent probes made from L. major Friedlin RNA from five time-points during differentiation from procyclics to metacyclics. The data were normalized for background and probe intensity and the relative abundance of RNA for each spot was calculated. Almost 15% (1387/9282) of the DNAs showed statistically significant (P<0.01) changes in expression (1.1-5-fold) during the transition, with 1.16% (108) showing up-regulation at two or more time-points and 0.14% (13) showing down-regulation. Northern blot analyses of selected genes confirmed these results. These studies confirmed the stage-specific expression of several known genes, as well as identifying a number of novel genes that are up-regulated in either procyclics or metacyclics.