Factor XIII is a proenzyme found in plasma and platelets which, when activated, catalyses the formation of ε-(γ-glutamyl)lysyl bonds in fibrin. Plasma factor XIII is composed of two different polypeptides, a and b, whereas platelet factor XIII has only a subunit. Factor XIIIa, the active enzyme, from both plasma and platelets is enzymatically the same. Platelet factor XIII is synthesized in megakaryocytes and accounts for 50% of the total factor XIII activity in blood. In this study the role of platelet factor XIII in the formation of plasma factor XIII was investigated. The chemotherapeutic agents—quinidine, vincristine, methotrexate and cyclophosphamide—were used to induce thrombocytopenia in rats. Platelet count, plasma factor XIII activity, and fibrinogen concentration were determined along with other haematological parameters. With all four drugs there were concomitant decreases in plasma factor XIII activity along with decreases in the platelet count. Throughout the experiment, fibrinogen concentrations remained within pretreatment levels. In the saline-treated control animals, the platelet count and plasma factor XIII activity did not significantly differ from the pre-treatment levels. These observations suggest that the low levels of factor XIII achieved in the treated groups resulted from the effects of the chemotherapeutic agents on thrombocytopoiesis. Statistical analyses of the data indicated a high correlation between platelet count and plasma factor XIII activity. From this study, it is apparent that platelet factor XIII could be a source of a subunit for plasma factor XIII.