The laboratory tests which should be at the command of every practitioner of medicine are those which deal with the more important and practically useful examinations of the blood, urine, feces, gastric contents, spinal fluids, pleural and ascitic fluids. These are the tests that are customarily taught in the medical schools in the course in clinical pathology, and the instruction is usually designed to take up the laboratory methods that are absolutely necessary for good practice and those only. An experience in teaching this subject during the last seven years has emphasized the striking fact that in spite of the great contributions which the laboratory has made to clinical medicine there has been surprisingly little change in the character or number of the technical methods which are essential for good practice. In many instances the progress of medical science has resulted in a clearer, broader, and more helpful interpretation of the tests, but the actual technical procedures have not been greatly altered and they are still available to the trained man who has a minimum of laboratory apparatus. It has, indeed, been interesting to find how little new material in the way of technical procedure could justifiably be added to the course from year to year, even though the literature and the practices of various clinics were carefully followed in the attempt to keep the course up to date. The methods for the examination of the urine, for instance, are taught much as they were two decades and more ago. Certain tests, such as urea determinations, have been discarded and others are regarded as having a different significance, but the records still show the color, specific gravity, reaction, albumin and sugar content, and the microscopic examination of the sediment. These simple observations, correctly used and interpreted, are practically all that is necessary in cases of nephritis. The modern “two hour renal test” requires nothing more than determinations of volume and specific gravity, and if it is combined with the phenolsulphonephthalein test--the technique of which is entirely simple--the field is open for the study of renal function. It is far more important to understand the significance of these easy tests than it is to worry about the quantitation of blood urea or blood uric acid. The situation is much the same with regard to hematology. The technical procedures of primary value are now as they have been for years, the counting of white cells and red cells, the estimating of hemoglobin, and the preparation of stained specimens of blood. Quite recently the students at the Harvard Medical School have also been instructed in the methods of counting platelets and of staining reticulated cells, but neither of these procedures involves any essentially new technique. With these, and one or two other tests, such as coagulation time and bleeding time, the field of hematology is open. Again, the technique has been altered but