Studies of canine erythropoiesis in vitro have been limited to a relatively mature class of precursor that gives rise to erythroid colonies of 8-64 cells after 2-4 days in culture. In this report we provide data indicating that a second class of erythroid precursor that gives rise to clusters of colonies or bursts can be grown from both dog marrow and peripheral blood. Dog bursts reach maximum size after 7-14 days in culture and have an appearance characteristic of human erythroid bursts. Dog burst forming units, or BFUE, are low density cells (less than or equal to 1.065 g/ml) that can be separated from the majority of colony forming units, or CFUE, by density centrifugation.