Samples of peripheral blood collected from healthy young American alligators (Alligator mississippiensis) were studied to determine baseline values and characterize cell types. Mean total leukocyte counts were 6.4 +/- 2.9 X 10(3)/mm3, with mean differential values of 54.7 +/- 47.5% heterophils, 10.4 +/- 6.0% eosinophils, 12.7 +/- 16.8% basophils, 23.9 +/- 4.9% lymphocytes, and 0.7 +/- 0.5% monocytes. Mean total thrombocyte counts were 23.0 +/- 7.0 X 10(3)/mm3, and mean total erythrocyte counts were 3.84 +/- 8.7 X 10(5)/mm3. Morphologic descriptions of Wright-Giemsa-stained alligator blood cells generally corresponded to those of their counterparts in other reptiles, birds, and mammals. Cytochemical tests were positive as follows: chloroacetate esterase-lymphocytes and monocytes; nonspecific esterase-heterophils and monocytes; acid phosphatase-heterophils, basophils, and monocytes; and alkaline phosphatase-heterophils, eosinophils, and monocytes. Phagocytic and microbiocidal capacity was tested by adding live Staphylococcus aureus to whole blood. Subsequent staining with acridine orange and observation with fluorescence microscopy showed in vitro phagocytic and microbiocidal activity of monocytes, heterophils, and to a lesser degree, eosinophils.