This study was conducted with rural Guatemalan children who were on the average markedly retarded in their growth and whose intake of protein of animal origin was extremely low. The circumstances were favorable to the demonstration of any effect vitamin B12 may have on growth and appetite. The daily oral administration of 20 micrograms of vitamin B12 in controlled trials involving 50 preschool children in one village and 228 school children in four rural schools did not produce any effect on their rates of gain in either height or weight. The study extended over a period of more than a year.