McAbPD4 could inhibit the proliferation of transformed cells Rat3-3 with an inhibition rate of 72.3% in vitro. Moreover, McAbPD4 decreased the Rat3-3's colony forming ability on soft agar and tumorigenicity in nude mice. By Flow Cytometry, it was indicated that the inhibitory effect was due to a blockade between G1 and S phase. It was shown by Western Blotting that the McAbPD4-defined antigen was a 40 KD protein, named p40, which was located on the membrane of target cells MGC803 and transformed cells Rat3-3 by immunofluorescence assay. The antigen p40 was highly expressed on Ha-, Ki-, Nb- and Src oncogene transfected cells, but was undetectable or at the best only faintly on cells transformed with mos or raf. The antisense oligonucleotide AS-1 of Ha-ras distinctively inhibited the proliferation of Rat3-3, the level of Ha-ras oncogene product p21 and p40 were decreased by 47.8%, 36.5%, and 52.0%, respectively. The inhibitory effect of AS-1 to GCM3T3 was more marked, each with 54.2%, 47.9% and 65.9%. It is suggested that p40 not only is a tumor-associated antigen, but also a transformation-related cell surface protein, which is closely associated with and might be regulated by the activation of ras or Src oncogene and its product. For the inhibitory effect of McAbPD4 to Rat3-3 cells, it is deduced that p40 might be a growth factor receptor mediating autocrine growth of Ha-ras transformed cells.