Hybrid clones derived from the fusion of normal and Rous sarcoma virus-transformed 3T3 fibroblasts were analyzed for fibronectin and laminin pattern of expression in order to find a possible correlation with tumorigenicity. Both organization in the pericellular matrix and secretion into the culture media were investigated by immunofluorescence and ELISA techniques. No significant difference in fibronectin or laminin release was found among hybrid clones exhibiting different levels of tumorigenicity. In contrast, distinctive immunofluorescence patterns of concomitant presence of low levels of fibronectin and high levels of laminin were constantly observed in all the tumorigenic clones.