The gel double-diffusion test, using a phage lysate antigen of M ycobactirium smegmatis, was studied for its application to the testing of bovine serum for tuberculosis. Factors studied for their effect on this system were time and temperature of incubation, pH, and the addition of the electrolytes sodium chloride and calcium chloride to the gel. Among the factors studied, the incorporation of 18% Na Cl into the gel appeared the most beneficial. Because the serum of a nontuberculous animal remained negative, it was believed that the increase in precipitation represented an increased antigen and antibody activity. It is possible that the aggregates formed in the presence of high concentrations of Na Cl may actually have contained less antibody than that formed in the isotonic medium. The function of Na Cl may have been a mechanical one, in which it provided nuclei for aggregation. Whatever the mechanism, in an evaluation of the serum of a calf experimentally infected with M. bovis, there appeared to be a direct relationship between the concentration of Na Cl in the gel and the density of the precipitin bands. As a result of these studies, the addition of 18% Na Cl, a pH of 5.0, and incubation at 23[degree]C. for 7 days were considered optimal and were adopted for later and more extensive evaluations of the serum of experimentally and naturally infected cattle.