1. Simple and rapid thin-layer and micro-thin-layer chromatography techniques are described for the detection of methylmalonic acid in urine. 2. The separation of methylmalonic acid from a crude urine sample is performed by thin-layer chromatography with a mixture of silica gel-cellulose and butanol - acetic acid - water solvent system. The methylmalonic acid spot is visualized with tetrazotized o-dianisidine. 3. This system has been successfully developed for a urinary screening programme; it was shown as simple, convenient and rapid, eliminating false-positives and allowing the detection of even traces of methylmalonic acid.