Abstract The procedure of diluting serum with distilled water was carried out and the resulting turbidity was read photometrically. This dilution turbidity test was compared with Maclagan's thymol turbidity test and Hanger's cephalin-cholesterol flocculation test. It was noted that at a dilution of 1:15 the dilution turbidity closely parallels the thymol turbidity. Experimental evidence confirms this observation of a close relation between these two tests; the technique of the procedure, the behavior of the dilution turbidity toward water and sodium chloride, its values before and after lipid extraction of the sera, its behavior with rabbits' blood and human immune serum globulin, and the fact that it flocculates within twenty-four hours when present in increased amounts all put the dilution turbidity test beside the thymol turbidity test rather than the cephalin-cholesterol flocculation test. Conforming with evidence obtained by others, a certain difference in mechanism between the thymol turbidity test and the cephalin-cholesterol flocculation test also is confirmed by our experiments and clinical experience. There is, however, an obvious difference in the results of the thymol and dilution tests in a small number of cases as well as in the experimental results. The discrepancies between the two tests as observed in clinical cases usually fall into the categories of neoplastic disease, congestive heart failure, and diabetes. The serum components responsible for the two respective tests therefore cannot be identical, but may overlap. The dilution turbidity test may serve as a simple test apparently giving most of the information supplied by the thymol turbidity test. It may, furthermore, confirm and occasionally modify the result obtained in a thymol test.