Serum vitamin B12 levels were assayed in 25 healthy and 35 protein-malnourished children aged 1 -6 years. Serum proteins and haematocrit values of the children were also measured. Liver enlargement was estimated clinically. The mean serum vitamin B12 activity among protein-malnourished children was slightly higher than that for the control group. Serum vitamin B12 activity of the healthy Nigerian children was, however, high, and possible reasons for this are discussed in detail. There was some correlation between very high serum vitamin B12 levels among the protein-malnourished children and incidence of liver enlargement.