Uridine-specific IgG and IgM antibodies, obtained by immunization of rabbits with a uridylated synthetic polypeptide, were isolated and purified by the use of a uridine-acetyl cellulose conjugate as immunosorbent. The IgM antibodies were separated from the IgG antibodies by gel filtration on Sephadex G-200. Both antibody preparations were pure by ultracentrifugation and immunoelectrophoresis criteria. The precipitating activities of IgM and IgG antibodies were similar on a weight basis, although the equivalence zone in the case of IgM antibody is broader. I n spite of its inability to precipitate the homologous antigen, the reduced alkylated IgM subunits were still able to inhibit specifically the homologous precipitation in the U-pAla-pLys -IgG antibody system. The average association constants for uridine of both IgG and IgM a.ntibodies, measured by the equilibrium dialysis technique, were low and of a similar order of magnitude.The complement fixing ability of purified IgM and IgG anti-uridine antibodies upon reaction with the homologous antigen displayed significant differences : low complement fixation activity was found in the IgM fraction either a t 4" or a t 37", while a high activity was found in the IgG fraction a t both temperatures. The IgM fraction lost all of its ability to fix complement when the macroglobulin was reduced and carboxymethylated. PCA reaction, using a highly immunospecifically purified I9 S anti-uridine antibody, was found to be negative when assayed up to an amount of 100 pg.