The concentrations of C-reactive protein (CRP) in serum from dogs diagnosed as normal by clinical, haematological, and biochemical examination were determined by enzyme-linked immunosorbent assay (ELISA) and slide reversed passive latex agglutination (RPLA), using IgG antibody isolated from rabbit anti-canine CRP serum. The mean value of CRP in 66 normal dogs kept in private households was 8.4 +/- 4.9 micrograms/ml by ELISA and 8.5 +/- 6.3 micrograms/ml by RPLA. Thus, no significant difference was demonstrated between the values obtained by ELISA and RPLA. No significant age and sex-related differences were found in the CRP values. The mean concentration of CRP in 84 6-month-old Beagle dogs kept in kennels by breeders was 6.2 +/- 3.9 micrograms/ml by ELISA and 8.0 +/- 4.0 micrograms/ml by RPLA. Again no significant difference was found. The CRP values determined by ELISA and RPLA were closely correlated (r = 0.913). The serum and plasma concentrations of CRP measured by RPLA were also closely correlated (r = 0.994). This indicates that plasma can be used as well as serum to determine CRP concentrations by RPLA.