Human diploid fibroblasts display a limited proliferative life span in vitro, which is directly correlated to the age of the donor from which the cells were explanted. In an effort to identify molecular events that may underlie the loss of proliferative potential in aging fibroblasts, we have determined, at the protein level, the abundance of several cell-cycle-regulated proteins and the activity of the two major members of the activator protein-1 (AP-1) DNA binding complex. We find that cyclin A and p34cdc2 expression is decreased by two- to four-fold in old fibroblasts, but that Fos expression and binding activity are reduced by as much as 95% in old, as opposed to young cells, despite equivalent amounts of p105Rb and Jun proteins being expressed. We have further determined that the composition of the protein complex which binds a consensus (-TGACTCA-) AP-1 site changes dramatically during in vitro aging. Since we have shown previously that AP-1 activity is required for progression through the cell cycle, we propose that the quantitative and qualitative changes seen in AP-1 may play a direct role in the gradual loss of proliferative ability seen as cells approach senescence.