The concentration of C-reactive protein (CRP) was measured in effusions from 50 dogs to assess the potential for measuring this protein to differentiate body cavity fluids. The effusions were classified as either transudates, modified transudates or exudates according to their total protein concentration, total nucleated cell count, cytological findings and aetiology, and the concentration of CRP was determined by a time-resolved immunofluorometric assay. There were significant differences between the concentrations of CRP in the three types of effusion; the highest concentrations were observed in the exudates (4·47 to 54·59 µg/ml), the lowest were in the transudates (0·0094 to 7·87 µg/ml), and the modified transudates contained intermediate concentrations of CRP (0·045 to 10·78 µg/ml). A cut-off value of 4 µg/ml had a sensitivity of 100 per cent and a specificity of 94·4 per cent for differentiating transudates from exudates, and a cut-off value of 11 µg/ml had a sensitivity of 88·2 per cent and a specificity of 100 per cent for distinguishing modified transudates from exudates. However, a cut-off value of 1 µg/ml had a lower sensitivity (80 per cent) and an unacceptably low specificity (66·7 per cent) for differentiating transudates from modified transudates.