Vitamin B12 binding proteins were separated into 2 peaks corresponding to small (TCS) and large (TCL) molecular weight fractions by gel-filtration of Sephadex G-200 using 0.005 M-sodium phosphate buffer, pH 7.4 containing 1 M-NaC1. Each peak, after dialysis and concentration, was chromatographed on DEAE-cellulose. 3 peaks of radioactivity were recovered from both TCS and TCL. Peaks from TCS had an apparent mol. wt. of about 40,000 and those from TCL about 110,000, as determined by gel-filtration on Sephadex G-200. On electrophoresis, peaks eluted with 0.06 M-phosphate buffer, pH 5.85 from both TCS and TCL moved as beta-globulins; those eluted with 0.1 M buffer, pH 5.8 between beta- and alpha 2-globulin and those eluted with 0.25 M buffer, pH 5.4 between alpha 1-globulin. In our assay system, TCS delivered 57CoB12 to L-1210 leukaemic lymphoblasts while TCL had no such activity. Of the 6 binders from DEAE-cellulose, only peaks eluted with 0.06 M and 0.1 M buffers from TCS delivered labelled B12 to these cells. Antisera prepared against TC II B12 reacted only with TC II B12 from serum, TCS, and fractions D and E obtained from TCS which were eluted with 0.06 and 0.1 M buffers, respectively.