An improvement in the results of treatment for malignant tumor in the oral cavity is expectd with combination of radiotherapy and chemotherapy. It is the purpose of this paper to report the time factor for the combined effects of X-ray and antitumor agents. A study has been conducted in vitro on cultured KB cells from the two stand points, namely the cell survival rate and morphologic changes. The results obtained are as follows : 1) In 5-Fluorouracil (5-FU) or Mitomycin C (MMC) treatment to KB cells, given before or after X-ray radiation, only additive inhibitory effects are shown, regardless of numbers of treatment of 5-FU or MMC. 2) In combined treatment of 5-FU or MMC with X-ray radiation for KB cells simultaneously, geometrical inhibitory effects are observed. 3) From the morphologic studies, the most intensive inhibitory effects are seen in the experimental series of simultaneous treatment of 5-FU or MMC and X-ray radiation. 4) As far as our expreimental series are concerned, no significant difference in inhibitory effects are noted by combined treatment of X-ray radiation with either 5-FU or MMC.