A new hypothesis regarding the effect of vitamin K 3 On hepatoma cell growth is presented. In brief, exploration of cell growth activity has been identified with the action of p34 c d c 2 kinase and its associated protein tyrosine phosphatase. After exploring a series of substituted derivatives of vitamin K and vitamin K 3 oxide, we suggest a mechanism involving alkylation at the active-site cysteine for the inhibition of the protein tyrosine phosphatase which controls the activity of the p34 c d c 2 kinase.