Reference curves demonstrating the relationship between serum or liver vitamin B12 and weight gain were derived from the examination of 16 published and 48 unpublished N.Z. trials. From these curves probability of obtaining an economic reponse (>10g/day body weight increase) for any serum or liver vitamin B12 can be determined. No significant (P<0.05) weight gain responses occurred to vitamin B12 or cobalt treatment in trials with mean serum vitamin B12 levels above 500 pmol/l or liver vitamin B12 levels greater than 500 nmol/kg. The reference curves were therefore derived from trials with vitamin B12 levels below these levels; 36 trials with serum vitamin B12 and 19 trials with liver vitamin B12 data. The mean vitamin B12 level at the mid point of the weight gain response period was selected from each trial.