In view of the unsatisfactory response to iron of certain common anaemias in childhood, it was decided to treat a series of cases with cobalt to find out if this substance would prove a satisfactory substitute for iron in refractory cases. Fifty cases were treated, and all had types of anaemia usually showing a poor response to iron, liver and other haemopoietic agents in common use. There were 31 cases of anaemia associated with infection, 10 were anaemic premature infants and three were cases of rhesus incompatibility after the need for transfusion had passed but which showed a residual anaemia. There was one case of severe physiological anaemia and one which developed anaemia after a Rammstedt's operation, although there was no evidence of gastric haemorrhage. The remaining cases comprised twins with a fairly severe anaemia of unknown aetiology and a case of erythrogenesis imperfecta which is included out of interest in spite of the baby's failure to respond to cobalt.