A commercial nephelometric test kit for human fibronectin (FN) was found suitable for the estimation of fibronectin concentration in body effusions of cats and dogs. The FN measurements were set in relation to the FN concentration of plasma pools in cats and dogs. A discrimination line of 31.5% completely separated malignant from cardiogenic pleural effusions in cats. For the diagnosis of a malignant pleural effusion, sensitivity was 100% and specificity was 57%. Pleuritis also resulted in high FN concentrations. The FN concentration in malignant pleural effusions in dogs differed significantly (P < .02) from that in cardiogenic effusions. There were no clinically useful differences in the FN concentration in peritoneal effusions in cats and dogs. The FN/albumin ratio was significantly higher (P < .02) in dogs with neoplastic abdominal effusion than in those with congestive heart failure.