The card agglutination test was used to examine serum samples from 246 buffaloes at Vitas Slaughterhouse, Tondo, Manila for the presence of antibodies to A. marginale. Antibodies were detected in 16.7% of sera collected from slaughtered animals originating from 5 of 6 localities in the country. Highest proportion of antibody-positive buffaloes belonged to the 5-6 year-old age group (21.7%) and the lowest, in >10 year age group (13.3%) but the difference was not statistically significant. Seropositivity was higher among males (17.8 %) than among female buffaloes (13.1 %), although the difference was not statistically significant. The performance of CAT was compared with blood smear method. Only a few positive reactors (6 of 24) had patent-parasifemia as revealed by Giemsa-stained blood smears. These results suggest that water buffaloes were commonly exposed to A. marginale and that CAT may be useful to identify infection in clinically normal water buffaloes.