Hematological studies have been carried out on a group of 96 common marmosets (Callithrix jacchus) on which full clinical histories were available. The group included both healthy and sick animals. Reference values for red-cell, white-cell, and platelet counts; hemoglobin levels; packed cell volumes; red- and white-cell absolute values; reticulocyte counts; and fibrinogen levels were calculated from results obtained in 43 clinically normal individuals. These reference values have been used to identify hematological abnormalities in sick animals so that relationships between the blood picture and the clinical findings could be defined. Leucopenia, raised platelet counts, and high fibrinogen levels were found in animals with bacterial infections and carried poor prognosis. Heinz bodies occurred in a significant number of the group as a whole, and were associated with a variety of hematological and clinical abnormalities. These findings illustrate the diagnostic and prognostic value of clinical hematology in C. jacchus.