We have devised a rapid and inexpensive gel filtration method to separate IgM fractions from small amounts of serum. These fractions have been titrated with IgM-IFAT and IgM-IHAT, and their titers have been compared with those obtained on whole serum. The results show that the separation of IgM from IgG allows detection of IgM antibodies in many cases of acquired toxoplasmosis, especially in cases of congenital toxoplasmosis. False positive results due to the presence of rheumatoid factors are also avoided. The IHA test performed on IgM fractions is constantly negative.