Antisera to ox red blood cells were prepared by injection of ox red blood cell stroma without adjuvant in outbred white rabbits. Purified IgM fraction for T-cell subset assay was obtained from these rabbit anti-ox red blood cell stroma antisera by precipitation with 50% saturated ammonium sulphate followed by DEAE-Cellulose chromatography and Sephadex G-200 gel filtration. The serological identification of purified IgM fraction was achieved by immunoelectrophoresis with guinea pig antiserum against rabbit anti-ox red blood cell IgM antibody.