Differential white blood cell counts were recorded for adult mallards (Anas platyrhynchos) of both sexes during several stages of reproduction: pre-egg laying, egg laying, incubating, molting, and postreproductive. Similar counts were made for young birds from 5 to 60 days of age. No significant (P less than or equal to 0.05) differences amongst the cell ratios due to sex or reproductive state of the adult birds were noted. Nonlaying and laying birds had similar numbers of thrombocytes which were significantly greater than thrombocyte numbers of incubating, molting or postreproductive birds. Young birds had a decrease in the percent lymphocytes from 50 to greater than 60 days of age and a concomitant, compensating increase in percent heterophils. Thrombocyte numbers increased from 5 days of age to a peak at 18 days of age, after which they did not vary significantly.