1. Caffeine N3-demethylation, the major pathway of caffeine metabolism in man, is mediated by P4501A2. The carbon of the methyl group lost during N3-demethylation is eliminated as carbon dioxide in vivo, or as formaldehyde and formic acid in vitro. 2. A simple and sensitive assay was developed to quantify the [14C]formaldehyde/[14C]formic acid produced following incubation of human microsomes with [3-14C-methyl]caffeine. This assay, using solid-phase extraction, enables quantitation of [14C]formaldehyde/[14C]formic acid with acceptable precision (within 5%) and accuracy (within 10%). 3. Typical Km and Vmax for the N3-demethylation of caffeine were determined by this assay to be 500 (range 220-1200) microM, and 250 (range 115-450) pmol.mg protein-1.min-1 respectively. 4. The N3-demethylation activity determined in microsomes from a range of human livers correlated significantly with other P4501A2 activities (p < 0.001) and was inhibited (> 95%) by furafylline. In addition, caffeine N3-demethylation was catalysed by microsomes from cell lines transfected with human P4501A2 cDNA. 5. This assay, for quantitation of [14C]formaldehyde/[14C]formic acid in human liver microsomes, is suitable for use in in vitro drug interaction studies as a probe for P4501A2 activity.