In order to test if there is an alteration in major surface proteins in human fibroblasts as they become senescent in vitro, activity of specific antisera against presenescent and senescent cells was measured. Two strains of human foreskin fibroblasts were grown into senescence by serial transfers. One strain (HF-J) became senescent after 49 population doublings while the second (HF-4) became senescent after 62. Antibodies were made against these cells while in the presenescent (phase II) and senescent (phase III) stages. Antibody binding to presenescent and senescent cells was measured before and after preabsorption with heterologous cells (e.g., presenescent HF-4 cell stimulated antisera was absorbed with senescent HF-4 cells, etc.). Two assays were used to measure antibody binding: complement mediated cell lysis and the binding of radiolabeled staphylococcal protein A. The amount of protein A binding after treatment with specific antisera was found to be the same for both senescent and presenescent cells. Likewise no difference in complement mediated cell lysis titers were observed. These results are consistent with the conclusion that senescent and presenescent cells do not differ in major cell surface antigens.