Modulation of gap junctional communication (GJIC) is likely to play an important role in tumorigenesis, as suggested by the action of tumor promoters and certain oncogene products. In this report we examine the effects of ras transformation and TPA (12-O-tetradecanoylphorbol-13-acetate) treatment on GJIC of murine primary keratinocytes. Introduction of the ras oncogene into primary keratinocyte cultures by Harvey Sarcoma virus (HaSV) infection is sufficient to cause a 70-80% reduction in their GJIC as measured by Scrape-Loading/Dye Transfer technique. Furthermore, while a 100% increase in GJIC is observed when normal keratinocyte cultures are induced to differentiate by addition of calcium, no such increase can be detected with their ras transformed counterparts. As with ras, TPA treatment of normal keratinocytes results in a 70-80% reduction of GJIC both under low and high calcium conditions. TPA treatment of keratinocytes already transformed by ras completely abolishes GJIC of these cells, regardless of calcium concentrations. The similar and synergistic effects of ras and TPA on GJIC of primary keratinocytes suggest that inhibition of this function represents an important early step in transformation of these cells.