A rapid fluorometric cellular adhesion assay in microtiter plates was evaluated for canine polymorphonuclear neutrophil granulocytes (PMN) using the fluorescent indicator calcein acetoxymethyl ester (Calcein AM). Optimum adhesive responsiveness occurred in bovine serum albumin (BSA)-coated plates after 30 min incubation at 37°C with 5 x 10⁵ PMN in 100 μl per well using phorbol myristate acetate (PMA, 100 ng/ml) as stimulant. The adhesive responsiveness for 15 laboratory beagle dogs was determined and the mean percentage of adherence for unstimulated (2.9%) and stimulated (74.3%) PMN and corresponding 95% confidence intervals was determined. The mean intra- assay coefficient of variation (CV(intra)) for unstimulated and stimulated adhesion in the assay was: CV(intra) = 12.9% and CV(intra) = 4.1%, respectively. The mean inter-assay coefficient of variation (CV(inter)) for unstimulated and stimulated adhesion, respectively, in the assay was: CV(inter) = 22.9% and CV(inter) = 5.6%. The assay was highly reproducible and labelling with Calcein AM neither reduced PMN viability nor activated the PMN.