cDNAs prepared from total RNA extracted from plaques of multiple sclerosis were amplified by the polymerase chain reaction. The 11-bp degenerate primers used were derived from conserved sequences of reverse transcriptase. Amplified cDNAs were fractionated according to size by electrophoresis in polyacrylamide gels under denaturing conditions. cDNAs of the proper size were cloned, grouped according to the sequence of their insert by differential hybridization, and sequenced. Six cDNAs were isolated and found to belong to new members of two groups of human endogenous retroviruses: the group related to ERV9 and that related to HERVK10 and HUMMTV. These sequences were expressed in all human organs tested, including normal white matter of brain. The approach described in this article is a powerful tool with which to isolate new members of the reverse transcriptase gene family.