A radio thin-layer chromatographic method is described for in vitro measurement of tolbutamide methylhydroxylation as an alternative to the commonly used HPLC assay. After the incubation experiments of [14C]tolbutamide with human liver microsomes, the supernatants were directly spotted onto standard silica gel TLC plates and developed in a horizontal chamber using a solvent system consisting of toluene-acetone-formic acid (60:39:1, v/v). Dried TLC plates were exposed to a phosphor imager plate and quantificated by use of a phosphor imager. Reaction rates were calculated from the ratio of labelled metabolite to the total radioactivity. The correlation coefficient between HPLC and the TLC method was 0.978 (n=14). The described method provides a valuable tool for the determination of tolbutamide hydroxylation activity in human liver microsomes.