In this communication, we have studied a 72,000 mol w (p72) host protein which reacts with a mouse monoclonal antibody (PAb6) directed against antigenic determinants on the Simian virus 40 (SV40) large T antigen protein that map 5' of 0.42 map units on the viral genome. The p72 protein is an abundant basic (pI greater than 7) cytoplasmic protein found in both SV40-transformed and untransformed parental cells and in cell lines derived from normal human and tumor tissue. By two-dimensional gel analysis and Western blot analysis the p72 protein identified by PAb6 is indistinguishable from the 72,000 mol w protein PABP associated with the poly(A)+ tract of cytoplasmic messenger RNA molecules. In normal human peripheral blood mononuclear cells stimulated to proliferate with the T-cell-specific mitogenic lectin phytohemagglutinin the synthesis and cytoplasmic accumulation of p72 occurs very early during the G0----G1-phase transition. The p72 protein is also expressed in proliferating and differentiated human promyelocytic HL60 cells indicating that the expression of this protein is not strictly limited to cycling cells.