The modulation of responsive genes by hormonal stimulation is an attractive in vitro model system for the study of a wide variety of biological processes. Using this methodology we have investigated the effect of the human oncogene protein p21ras on mitosis using mouse mammary tumor virus long terminal repeat (MMTV-LTR)-directed gene expression. Following the induction of p21 protein, abnormal mitotic figures were scored in metaphase and anaphase. Elevated expression of p21 was associated with marked increase in the proportion of abnormal mitoses most significantly during the metaphase. Concomitant with a three fold increase in p21 levels, abnormal mitosis rose from 14.0% to 27.25%. The increase in abnormal mitosis corresponded to a 225% increase in abnormal metaphase. The p21-induced mitotic abnormalities were exhibited as lagging chromosomes in prometaphase, 3 group metaphase and C-metaphase. In addition, high expression of p21 was accompanied by significant changes in the cell morphology and fine ultrastructure, e.g. disorganization of actin, the extensive formation of microvilli on the plasma membrane and marked dilatation of the rough endoplasmic reticulum. The mitotic and structural changes were reversible upon removal of dexamethasone and decline of p21 production to its basal levels. Our results identify an important biological effect of ras p21 during mitosis and the early stages of neoplastic transformation.