In order to study the possible role of intercellular communication in the process of in vitro cell transformation, the communicating capacity of BALB/c 3T3 cells transformed by 20-methylcholanthrene was investigated using a dye transfer method. Morphologically transformed foci, detectable 4 to 5 weeks after treatment with 20-methylcholanthrene (1.0 microgram/ml), are clearly distinguishable under the phase-contrast microscope from surrounding nontransformed monolayer cells; therefore, a tracer dye can be injected into individual cells, and gap-junctional communication between and among transformed and nontransformed can be studied directly. When fluorescent Lucifer Yellow CH was microinjected into a cell within the transformed focus, the dye was transferred to other transformed cells but not to cells in the adjacent nontransformed area, although they were in physical contact. Similarly, dye injected into a nontransformed cell was transferred to neighboring nontransformed cells but not to cells in an adjacent transformed focus. These results indicate that when BALB/c 3T3 cells are transformed by 20-methylcholanthrene they can no longer communicate with surrounding normal cells, although they maintain the ability to communicate with other transformed cells in the focus. These results suggest that loss of the ability to communicate with surrounding nontransformed cells may be one of the important determinants of induction and expression of the final malignant transformation.