The effect of experimental hematomas on serum levels of fibrinogen- related antigen (FRA) was studied in rabbits. Homologous blood used for production of hematomas was mixed with 131I-fibrinogen, and the measurement of FRA was made using a radioactive technique (representing FRA from hematoma) and immunologically (representing total FRA). Total body radiation of animals bearing hematomas containing 131I-fibrinogen decayed slowly (mean t 1/2 = 189 hr) over the first 2–3 days and then rapidly (mean t 1/2 = 43 hr) during the subsequent days. Serum FRA levels measured immunologically rose transiently from 4 to 11 mug/ml during the slow phase of hematoma resolution, and returned to baseline levels during the rapid phase of resolution. The appearance of a small amount of protein-bound radioactivity in the blood of animals with hematomas paralleled the time course of the changes in immunologically determined FRA, but, even at its peak, circulating levels only averaged 0.36% of the administered tracer. From these observations, it seemed unlikely that the dissolution of fibrin which may form as the result of bleeding into tissues would significatnly contribute to levels of circulating FRA.