The effect of cobalt on haemoglobin synthesis was studied in reticulocytes obtained from marmosets, new world monkeys. By means of different inhibitors the conditions necessary for either cobalt-induced stimulation or inhibition were determined. Cobalt-induced inhibition of globin synthesis was not reversible by the addition of either delta-amino levulinic acid or haem. Globia synthesis, however, in iron-deficient reticulocytes was stimulated by the addition of cobalt. Cobalt did not apparently require complexing with protoporphyrin for activity, since the effect was observed whether the haem-biosynthetic pathway was inhibited or not. The data have been interpreted to suggest cobalt operates by affecting a rate controlling step in the synthesis of globin.