Abstract: Method for predicting incipient failure of a lubricated engine by removing a sample of the liquid lubricant employed after the same has been in contact with the moving metal surfaces of the engine. The metal wear particles contained in the lubricant are then separated into at least three predetermined size ranges by passage through membranes having pore openings corresponding in size to such predetermined size. Each of the fractions so separated is analyzed for a selected metal contaminant, such as iron, and the content of such metal contaminant is plotted against the particle size thereof. The likelihood of mechanical failure is determined from the shape of the resulting graph, with a preponderance of large metal contaminant particles being indicative of incipient engine failure.