Bovine erythroid colonies containing eight to 100 erythroid cells were successfully cultured in methyl cellulose. The colonies required four to five days to mature and a direct linear relationship (r = 0.99) between the number of nucleated marrow cells plated and the number of colonies formed was demonstrated. Their optimum erythropoietin requirement was observed to be 1 U/ml. No second population of erythroid colonies developed either as a function of time or erythropoietin concentration. However, 1 to 5 per cent of the bovine erythroid colonies contained megakaryocytes in addition to the erythroid cells. Marrow samples from different cattle showed a wide variation in their response to erythropoietin in culture, while some produced many erythroid colonies in the absence of erythropoietin, others were observed to be entirely erythropoietin dependent. Large mixed colonies occasionally grew in methyl cellulose but their optimum culture conditions have not yet been established. Medium conditioned by phytohaemagglutinin-stimulated bovine peripheral leucocytes enhanced bovine erythroid colony growth. Bovine erythroid progenitor cells were observed to possess a sedimentation velocity of 9 +/- 1 mm per hour.