This unit describes methods for identifying bacteriophage or cosmid clones that contain sequences present in the mRNA of a cell line or tissue. In the , a radiolabeled cDNA probe is synthesized by reverse transcription of poly(A)+ RNA isolated from the cell line or tissue of interest. The probe is incubated with DNA-cellulose to remove highly repetitive sequences before it is used for hybridization analysis of filters from a phage or cosmid genomic library. After identification of positive clones from the library screen, the same probe can be used to screen Southern blots of restriction enzyme-digested DNA from the positive clones. Support protocols describe preparation and testing of DNA-cellulose.