Complete blood counts (CBCs) were performed on 68 fetal macaque blood samples collected from 31 fetuses by repeat ultrasound-guided cardiocentesis (gestational day [GD] 80–150; term ∼ GD 165) with the goal of defining normative hematologic reference values during prenatal life. Lymphocytes were the primary white blood cells (WBCs) observed throughout gestation. Segmented neutrophils were the second highest percent of WBCs and were noted to rise significantly during GD 130–140. Overall, WBCs increased progressively throughout gestation with a transient decline at the beginning of the third trimester (∼ GD 110). A steady linear rise in red blood cells (RBCs), hemoglobin, and hematocrit was observed with a marginal decline close to term; mean corpuscular hemoglobin (MCH) and mean corpuscular volume (MCV) showed a consistent linear decline throughout gestation whereas mean corpuscular hemoglobin concentration (MCHC) remained relatively stable. Nucleated RBCs (NRBCs per 100 WBCs) were present on GD 80, showed a sharp decline mid-second trimester (∼ GD 90), and were significantly diminished by the beginning of the third trimester (∼ GD 110). No quantitative changes in platelets were noted throughout the study period. Evaluations of erythrocyte morphology indicated polychromasia and anisocytosis as typical characteristics throughout gestation with rare occurrence of hypochromasia and poikilocytosis. Howell-Jolly bodies were intermittently observed. These data reveal distinct similarities when compared to prenatal hematologic characteristics for human fetuses and provide baseline data which can be applied to experimental studies where prenatal hematopoietic/hematologic toxicity may be anticipated. © 1993 Wiley-Liss, Inc.