A diploid cell strain of normal human endothelial cells, RuBa 7E was established and characterized. The toxic and mutagenic effects of N-methyl-N''-nitro-N-nitrosoguanidine on RuBa 7E cells were studied and are similar to those reported for diploid human fibroblasts. Mutant cells lacking hypoxanthine-guanine phosphoribosyltransferase were selected by their resistance to 6-thioguanine. The spontaneous incidence of mutants was .ltoreq. 6 .times. 10-6 and the induced incidence was 4.4 .times. 10-4 at a survival frequency of 0.05. All 17 mutants tested lacked detectable hypoxanthine-guanine phosphoribosyltransferase activity and none grew in medium containing azaserine and hypoxanthine. Mutant cells incorporated radioactive adenine but did not incorporate radioactive hypoxanthine. Unlike human fibroblasts, in which the recovery of 6-thioguanine-resistant mutants is reduced by contact feeding when the inoculum size during selection is increased above 104 cells per P60 dish, 5-10 .times. 104 RuBa 7E cells can be plated per P60 dish without reducing mutant recovery. RuBa 7E cells were infected with SV40. As early as 14 days postinfection, discrete foci of morphologically transformed, mitotically active cells were seen against a monolayer background of normal cells when cultures were maintained in medium with low serum. Of the 33 foci obtained, 7 were studied for SV40-specific viral T-antigen and all were positive. The facility with which RuBa 7E cells can be mutagenized and the ease with which morphological transformants can be identified make these cells potentially useful for studies comparing the mutagenic and transforming effects of chemicals and other agents on diploid human cells.