By culturing heparinized whole blood, satisfactory chromosome preparations have been obtained easily and efficiently from blood samples as small as 0.05 ml. This technique eliminates any special treatment of the blood prior to initiating culture and minimizes the loss of leukocytes from the sample. It is particularly useful in those situations where separation of the leukocytes from other blood cells is difficult. Chromosomes from humans and deer mice have been studied using this technique.