I have identified an activity in crude HeLa cell extracts that satlsfles the requirements for a human telomere terminal transfemse or telomemse. It catalyzes the addition of a 6 nucleotlde repsating pattern to ofigonucleotide primers containing human or nonhuman telomerit repeat sequences. Direct sequence analyses of reaction products reveal the added sequence to be TTAGGG in all cases. Under optimal conditions 65-70 repeats can be synthesized. The enzyme has the properties of a rfbonucleopmtein. Telomerase has previously been observed only in ciliated protozoans, which possess lOa-10’macronuclear telomeres. The identffication of telomemse in HeLa cells with only 400 telomeres indicates that telomerase-mediated telomere maintenance is conserved throughout eukaryotes.