A rapid method for the determination of lipid is described in which extraction of the lipid from the sample is carried out in a sealed tube by using a chloroform-water-methanol solvent system. The weight of lipid in a volumetric sample of the chloroform-lipid solution is determined after evaporation of the chloroform. Estimates of precision give coefficients of variation of 0·26 per cent. for recoveries of pure lipid, 0·56 per cent. for muscle samples and 0·89 per cent. for fat tissues. Comparison of fatty-acid recoveries in the extracts from chloroform-water-methanol extraction and from Soxhlet extraction with diethyl ether shows that the chloroform-water-methanol system is, in all the instances examined, equal or superior to the Soxhlet method in efficiency. The time required for one determination is about 1 hour, but an output of sixty determinations per operator per working day can be achieved without difficulty. The method is recommended as a routine laboratory procedure, particularly for materials that contain a high proportion of lipid in association with protein.