The protein-mapping method which combines isoelectric focusing in acrylamide gel and gel electrophoresis was previously used mainly for the separation of plant proteins and human serum proteins. We investigated with this technique soluble proteins of mouse tissues (whole embryos, the liver of fetal and adult mice, kidneys) and the proteins of mouse serum. The technique was tested under a number of different conditions to find those best for our purpose; they may represent some general improvements in the method. The protein patterns show high resolution and excellent reproducibility. About 275 spots were found for fetal liver, about 230 for whole embryos (day 14 p.c.) and about 100 for serum. The fact that a high number of protein spots can be evaluated by a single and comparatively simple experiment suggests that this method may be useful as an assay system for induced point mutations. The protein patterns demonstrated are compared and discgs of dominant lethal examinations after acute and subacute application of these three substances.