Three parameters were evaluated as diagnostic of the malignant potential of cultured rat liver epithelial cells: cytology, growth in soft agar, and production of extracellular plasminogen activator. A total of 22 tumorigenic and nontumorigenic cultures from 15 cell lines were sent coded from their originators to two different laboratories for the evaluation of these three parameters. Cytologic diagnosis and growth in soft agar were reliable means of determining the malignant potential of the cultured cells. However, the production of extracellular plasminogen activator showed little correlation with tumorigenicity. Of cytologic properties evaluated, the two that correlated best with malignant potential were increased cytoplasmic basophilia and and increased nuclear:cytoplasmic ratio.