The effective dose, schedule, molecular basis of the cytotoxicity of taxol and their dependence on the genetic background in tumor cells are still not well understood. Here, we examined how the dose-response relationship for taxol varies in lung cancer cells with different p53 status and under isogenic conditions. DNA content analyses in A 549 (p53, +/+) and H 1299 (p53, -/-) cells, showed that taxol progressively induced G2/M arrest in both cell lines in a concentration-dependent manner, which was accompanied by a parallel decrease in the G1 population. G2/M arrest, however, occurred at a lower concentration in A 549 cell lines than in H 1299 cells. The S-phase population in A 549 cells was not significantly changed up to 0.025 microM, but dropped by six-fold at 1.0 microM taxol, in contrast to that in H 1299 cells. A sub-G1 apoptotic population was present at 24 h, even at 0.002 microM taxol, when G2/M arrest was not appreciably detected. In both cell lines, the maximum apoptosis of about 28% was achieved at 0.025 microM taxol, implicating that wild-type p53 does not modulate the level of taxol-induced apoptosis. When we examined the role of the wild-type p53 in isogenic cell lines developed in a H 1299 background, the maximum level of apoptosis was in the range of 28-34% at a drug concentration around 0.03 microM, not significantly different from that observed in parental H 1299 cells. We conclude that taxol is effective in inducing apoptosis at very low doses (0.020-0.035 microM), and that the presence or absence of the wild-type p53 does not make a statistically significant difference in the level of apoptotic cell death in these lung cancer cell lines, but the maximum is attained at a lower drug concentration in the presence of p53.