The antitumor effects of 2 aromatic retinoids (analogs of vitamin A) were investigated in human bronchogenic carcinomas grown in vitro and in vivo (serially transplanted in "nude" mice). In vitro, bronchogenic carcinoma cells treated with Ro 10-1670 (the free acid of an aromatic retinoid) displayed activation of the lysosomal system and increased cellular breakdown. In vivo, nu/nu-mice treated with Ro 10-9359 (the ethyl-ester of Ro 10-1670 showed inhibition of tumor growth. In the tumors, necrotic areas were intermingled with regions displaying a higher degree of keratinization as compared to those of the controls. The extracellular matrix surrounding the tumor cells of the retinoid-treated animals was increased in volume and electron density.