LASER microprobes currently in use combine a laser microscope with a light emission spectrometer1-3. Recent improvements in instrumentation have demonstrated the limitations of this method with respect to spatial resolution (some µm) and detection capability (10-13-10-15g). Much lower detection limits (10-18 g) have been obtained in ultrathin sections with electron probe X-ray microanalysers4,5, although the latter, however, do not satisfy many needs of biologists. Neither trace element detection in the parts per 109 range nor analytical information about organic constituents can be obtained. In the case of the main physiological electrolytes, biologists are primarily interested in the cellular and subcellular transport kinetics of Na+, K+, Mg2+, and Ca2+, indicating a need for microprobe techniques capable of analysing isotopes. A mass spectrometer in combination with a laser microprobe seems to be a very attractive approach. In principle, it should enable not only high sensitivity but also isotope analysis and the `fingerprinting' of organic molecules, provided they do not decompose into fragments too small for recognition of the parent molecules. The special requirements for biomedical application are satisfied by the laser microprobe mass analyser (LAMMA) described here.