Thymoquinone (TQ), the most abundant constituent in black seed, was shown to possess potent chemopreventive activities against DMBA-initiated TPA-promoted skin tumors in mice. Despite the potential interest in TQ as a skin antineoplastic agent, its mechanism of action has not been examined yet. Using primary mouse keratinocytes, papilloma (SP-1) and spindle (I7) carcinoma cells, we studied the cellular and molecular events involved in TQ's antineoplastic activity. We show that non-cytotoxic concentrations of TQ reduce the proliferation of neoplastic keratinocytes by 50%. The sensitivity of cells to TQ treatment appears to be stage dependent such that papilloma cells are twice as sensitive to the growth inhibitory effects of TQ as the spindle cancer cells. TQ treatment of SP-1 cells induced G₀/G₁ cell-cycle arrest, which correlated with sharp increases in the expression of the cyclin-dependent kinase inhibitor p16 and a decrease in cyclin D₁ protein expression. TQ-induced growth inhibition in I7 cells by inducing G₂/M cell-cycle arrest, which was associated with an increase in the expression of the tumor suppressor protein p53 and a decrease in cyclin B₁ protein. At longer times of incubation, TQ induced apoptosis in both cell lines by remarkably increasing the ratio of Bax/Bcl-2 protein expression and decreasing Bcl-x_L protein. The apoptotic effects of TQ were more pronounced in SP-1 than in I7 cells. Collectively, these findings support a potential role for TQ as a chemopreventive agent, particularly at the early stages of skin tumorigenesis.