1,2-Dimethylthdrazine (DMH) has been known to induce vascular neoplasms, such as malignant endothelioma, in animal experiments through the induction of abnormal proliferation of human umbilical vein endothelial cells (HUVECs). We studied the effect of protein kinase C (PKC) isoforms on DMH-induced abnormal proliferation of vascular endothelium to identify the isoforms with higher relevance. The study was conducted with pure culture HUVECs in a control group and a 1x10-9 M DMH-treated group. The mRNA and protein expression of PKC isoforms in DMH-treated HUVECs was evaluated by reverse transcription-polymerase chain reaction and western blotting. DMH-induced PKC production was detected by a PKC activity assay. To investigate the role of the PKC isoforms in DMH-induced abnormal HUVEC proliferation, we modulated PKCµ expression in DMH-treated HUVECs using small interfering RNA. We determined the expression of 11 PKC isoforms by PCR analysis in both the control and DMH-treated groups, and the results were statistically analyzed to detect any differences. According to the results, both groups expressed 6 out of 11 isoforms. Expression of PKCµ was significant in the DMH-treated group, and downregulation of PKCµ inhibited DMH-induced abnormal HUVEC proliferation. The PKCµ isoform is believed to be important in the abnormal growth of vascular endothelial cells induced by DMH, and this was confirmed by an objective siRNA experiment, which showed a clear decrease in PKCµ expression. Therefore, it is believed that PKCµ is a key factor in the abnormal proliferation of vascular endothelial cells, and these results can be used as fundamental research data for abnormal vessel development or the embryologic mechanisms of vessel development.