Human foetal lung strain, MRC-5, was treated with simian virus 40 and cultures were obtained that had many of the properties of transformed populations. In 10 experiments, only two produced permanent lines, designated MRC-5V1 and MRC-5V2, which have grown to passage 750 and 650, respectively. In all cases, the SV40-treated cultures acquired many of the features of transformation, including production of T-antigen, loss of contact-inhibition, and ability to grow in low concentrations of serum. The presence or absence of other transformed characteristics, such as altered morphology, abnormal karyotype or ability to grow in soft agar, can be used to distinguish between individual newly infected cultures. However, the cells invariably entered a period of slow growth, or crisis, and in eight experiments the cultures subsequently died without the emergence of a permanent line. The report that late-passage diploid cultures ae more easily transformed to permanent lines than young cultures has not been confirmed. MRC-5V1 initially had a sub-diploid chromosome number, but during serial passaging this gradually increased. MRC-5V2, which has a more extreme transformed phenotype than MRC-5V1, had a hyper-diploid chromosome number, which also increased during long-term growth. MRC-5V1 became polymorphic for glucose-6-phosphate dehydrogenase, as judged by the heat-lability and electrophoretic mobility of the enzyme. Fusions between MRC-5V1 and Lesch-Nyhan fibroblasts yielded hybrids with a limited lifespan, and certain sub-lines of MRC-5V1 also slowed down, exhibited characteristic signs of senescence and ceased to grow.