A capillary reversed passive latex agglutination test (capillary RPLA) was developed which allows quantification of serum C-reactive protein (CRP) within approximately 15 min. The logarithmic regression line (calibration curve) obtained after measuring each CRP concentration three times in twofold dilutions of a standard canine serum containing 222 micrograms/ml of CRP was y = 6.394 + 0.030x (r = 0.995). Capillary RPLA permitted quantification of CRP in the range 6.9-222 micrograms/ml. The coefficients of variation ranged from 10.28% to 12.40%. The recovery rates (percentage recovery) of CRP by capillary RPLA were within the range 87% to 106%. On measuring the CRP concentrations in sera from 78 dogs by capillary RPLA, single radial immunodiffusion (SRID) and enzyme-linked immunosorbent assay (ELISA), close correlations were demonstrated between SRID and capillary RPLA (y = 7.250 + 1.109x, r = 0.978), between SRID and ELISA (y = 3.042 + 1.059x, r = 0.967), and between capillary RPLA and ELISA (y = 1.778 + 0.929x, r = 0.962). Capillary RPLA may be considered useful as a routine biochemical technique for measurement of serum CRP concentration in the dog.