Effects of Sterigmatocystin (ST) on ras p21 expression and ultrastructure of human lung fibroblasts in vitro were studied with immunological flow cytometry and electron microscopy. The results showed that ras p21 expression in the ST treated cells was dramatically increased and the ultrastructural features of malignant cells, such as enlarged nuclei with bizzar nuclear membranne folds and predominant nucleoli, could be seen.