The MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] assay is one of the most commonly used assays to assess cell proliferation and cytotoxicity, but is subject to interference by testing compounds. Hydralazine, an antihypertensive drug, is commonly investigated in multiple fields such as heart failure, cancer, and blood pressure research. This study reported interference of the MTS assay by hydralazine and a simple modification overcoming this interference. Vascular smooth muscle cells were cultured in the presence or absence of hydralazine (0, 10, 50,100, and 500 mu M) for 2 or 24 h. Cell numbers were analyzed using MTS, trypan blue exclusion, or microscopic assays. A modified version of the standard MTS assay was established, in which an additional step was added replacing the test medium, containing hydralazine, with fresh culture medium immediately before the addition of the MTS reagent. Culture with hydralazine at concentrations of 50, 100, and 500 mu M for 2 h increased absorbance (p< 0.05) in the standard MTS assay, whereas microscopy suggested no change in cell numbers. Culture with 500 mu m hydralazine for 24 h increased absorbance (p< 0.05) in the standard MTS assay, however, trypan blue exclusion and microscopy suggested a decrease in cell numbers. In a cell-free system, hydralazine (>= 10 mu M) increased absorbance in a concentration-dependent manner. The modified MTS assay produced results consistent with trypan blue exclusion and microscopy. In conclusion, a simple modification of the standard MTS assay overcame the interference of hydralazine and may be useful to avoid interference from other tested compounds.