Sephacryl S-300 has been compared with Sephadex G-200 in the fractionation of sera for the detection of rubella-specific IgM. No difference in sensitivity was found with sera that had low titres of rubella-specific IgM. Sephadex G-200 was apparently more sensitive with some sera which had high titres of rubella-specific IgM due to the contribution of rubella-specific IgA. Sephacryl S-300 offers considerable advantages in ease and speed of preparation, and as it is a more rigid gel higher flow rates can be obtained. Sera may be fractionated in 2.5 hours on Sephacryl S-300 compared with 7-10 hours on Sephadex G-200.