Normal human diploid fibroblast-like (HDF) cells exhibit a limited proliferative life-span when serially cultured. Limited in vitro replicative life spans have been described for a variety of cell types, and shown to be inversely related to donor age as well as directly related to the maximum life span of the donor species. Reproducible changes in gene expression accompany replicative senescence in HDF cells. We report the isolation of cDNAs for a number of genes that are differentially expressed between non-proliferating early (young) and late (senescent) population doubling level (PDL) WI-38 fetal lung-derived, HDF cells and characterize one of these isolates, named EPC-1 (Early PDL CDNA-1). EPC-1 derives from an approximately 1.4 kb mRNA species that is expressed in density-arrested and/or serum-starved young cells, but not in log-phase, low density, young cells. In contrast, EPC-1 transcript abundance and expression of the encoded protein are both negligible in senescent WI-38 cells under all culture conditions tested. Complete nucleotide sequence analysis of this CDNA confirms its identity with that of a cDNA encoding a secreted, retinal pigmented epithelium differentiation factor (PEDF) as well as similarity of the encoded protein with a number of mammalian serine protease inhibitors. EPC-1 is expressed in a variety of proliferative cell types, is encoded by a single copy gene in WI-38 cells, and has related genomic sequences conserved throughout a number of mammalian species and in the chicken. SV40-transformed WI-38 cells fail to accumulate EPC-1 transcript, but when its expression is exogenously enforced these cells have a reduced capacity to synthesize DNA. When antiserum or purified antibodies are used to functionally deplete secreted EPC-1, normal WI-38 cells have an increased capacity to synthesize DNA. Further, the decline in EPC-1 expression seen in proliferatively senescent cells in vitro is also displayed by fibroblasts derived from aged donors. These findings support the hypothesis that there is an association between EPC-1 and reversible growth arrest and that cells aged in vitro or in vivo exhibit a state of growth arrest fundamentally distinct from that of quiescent (G₀) young cells.