The author evaluated the effects of taxol, a microtubular inhibitor, as a possible radiation sensitizer and the production of prostaglandins on three human cancer cell lines(KB, RPMI-2650 and SW-13) and one murine cell line(L929). Each cell line was divided into four groups (control, taxol only, radiation only and combination of taxol and radiation). The treatment consisted of a single irradiation of 10Gy and graded doses (5, 50, 100, 200, 300, 500 nM) of taxol for a 24-h period. The cytotoxicity of taxol alone was measured at 1 day after(1-day group) and 4 days after(4-day group) the treatment. The survival ratio of cell was analyzed by MTT (3-(4,5-dimethylthiazol-2-yl) -2,5-dimethyl tetrazolium bromide) test. Prostaglandins(PGE2 and PGI2) were measured in the culture medium by a radioimmunoassay. The results obtained were as follows. 1. There was a significantly increased cytotoxicity of KB cells in 4-day group than those in I-day group. There was a high correlation between doses of taxol and cell viability in both groups(l-day group R=0.82741, 4-day group R=0.84655). 2. There was a significantly increased cytotoxicity of RPMI -2650 cells treated with high concentration of taxol in 4-day group than those in I-day group. Also there was a high correlation between doses of taxol and cell viability in 4-day group(R=0.93917). 3. There was a significantly increased cytotoxicity of SW-13 cells treated with high concentration of taxol in 4-day group than those in 1-day group. However no high correlation was observed between doses of taxol and cell viability in both groups(1-day group R=0.46362, 4-day group R=0.65425). 4. There was a significantly increased cytotoxicity of L929 cells treated with low concentration of taxol in 4-day group than those in 1-day group. At the same time, there was a low correlation between doses of taxol and cell viability in both groups(1-day group R=0.34237, 4-day group R=0.23381). 5. In I-day group of L929 cells, higher cytotoxicities were observed in the groups treated with 500 nM taxol than given 10 Gy radiation alone. L929 cells in I-day group alone showed a radiosensitizing effect by taxol.. 6. In addition to L929 cells, all cancer cells treated with a combination of taxol and radiation in 4-day group appeared to have some fragmented nuclei and to float on the medium. In addition, L929 cells appeared to be more confluent. 7. The level of PGE2 production was the highest in the contol KB cells. This appeared to increase in every experimental group of all three cancer cells except L929 cells. There was a significantly increased production of PGE2 in SW -13 cells treated with a combination taxol and radiation compared to the other experimental groups. 8. The level of PGE2 production in the control group of RPMI-Z650 cells was the highest. This appeared to increase in every experimental group of all cells except in SW-13 cells. This also increased significantly in RPMI-2650 cells treated with a combination of taxol and radiation compared to the other experimental groups.