A macroscopic (tube) complement fixation test for poliomyelitis, using infected tissue culture fluids, is described. The test was applied to 27 individuals with a clinical diagnosis of poliomyelitis. In 18 patients it was possible to make a laboratory diagnosis of poliomyelitis on the basis of a rise in complement-fixing antibody titer and in 4 others on the basis of a high stationary antibody titer. One individual gave a high and equal antibody response to two virus types, 3 others had no detectable antibody, and 1 appeared not to have poliomyelitis. Heterotypic reactions were encountered, but gave little difficulty in interpreting homologous responses. In those patients from whom a virus had been recovered, the serologic findings corresponded to the virus type recovered. The possible occurrence of dual infections with the viruses of poliomyelitis and Western equine and St. Louis encephalitis is discussed.