The determination of urinary 3-methyladenine has been explored as a potential monitor of exposure to methylating carcinogens. A method based on the use of high resolution gas chromatography--mass spectrometry has been developed for measuring this methylated base in human urine. Urine is extracted by XAD-2 column chromatography and the extract purified by reverse phase C18 h.p.l.c. Derivatization with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide yields the mono-tert-butyldimethylsilyl derivative of 3-methyladenine which has good gas chromatographic properties and is stable towards hydrolysis. The gas chromatographic separation was carried out using an SE-52 capillary column and quantitation made by electron impact selective ion monitoring mass spectrometry employing deuterium-labelled 3-methyladenine added initially to the urine as internal standard. Measurable levels (4.50-16.07 micrograms/24 h) of the methylated base have been found in control (i.e. nominally unexposed) human urines.