In chronic myelogenous leukemia both the serum concentration of vitamin B12 and the capacity of the serum to bind added vitamin B12 are markedly increased (1-3), suggesting an abnormality of the serum proteins in this disease. In vitro measurements of vitamin B12 binding capacity employing a microbiological assay method have not demonstrated a qualitative difference between normal and leukemic sera in respect to the binding of added vitamin B12 by the various serum protein fractions (1,4 of radioactivity in each sample determined by means of a plastic scintillation well counter. After one hour incubation at 220 C., the bags were dialyzed against 250 ml. of 1/15 M phosphate buffer, pH 7.3. Initially, the dialysis was carried on for 24 hours at 22°C. with constant agitation but subsequently this was performed for 48 hours at 40 C. without agitation. Equilibrium was attained in both situations, and similar values for vitamin B12 binding were obtained under both experimental conditions. At the conclusion of dialysis each bag was rinsed externally in running water for one-half to one minute after which its radioactivity was determined in the plastic scintillation well counter. The bags were then slit in two, emptied of their contents, washed for one-half to one minute in running water and recounted for the measurement of the residual radioactivity bound to the bag. The unsaturated vitamin B. binding capacity of the serum (UBBC) was determined from the following calculations:1. Fraction of added Co°B12 bound per ml. = (a) Co60B12 in bag postdialysis (cpm) -Co°B12 bound to bag (cpm) CoO°B12 added predialysis (cpm) X ml. serum -mean Co6B12 in blank calculated as in (a) 2. UBBC, m/Ag./ml. = Co°Big added predialysis (miAg.) X calculation 1.