This collection includes the IMR-90 (female) and IMR-91 (male), human fetal lung fibroblast lines which were developed expressly for the NIA Aging Cell Repository and are available at low, intermediate, and high population doublings. IMR-91 diploid male fetal skin fibroblasts cultures are also available at low, intermediate, and high population doublings. Included in this collection is WI-38 (AG06814 - female) which was the first diploid cell strain developed and utilized for the study of aging at the cellular level. WI-38 is available at population doublings levels 14, 22, 34, and 44. The Repository also distributes TIG-1 (AG06173 - female) which was developed in Japan for use in aging research. MRC-5 (AG05965 - male) from Britain, another well characterized diploid human fetal lung fibroblast cell strain, is available at three population doubling levels. All of these cultures have been widely used for aging studies and an extensive literature details the role they have played in research on the causes and effects of aging at the cellular level as well as many other biochemical studies.