A monoclonal antibody prepared against the eosinophil major basis protein (MBP) was compared with the anti-eosinophil cationic protein (ECP) antibodies (EG1 and EG2) in immunostaining of bronchial biopsies from atopic asthma and controls. Anti-MBP (designated BMK-13) did not cross-react with other eosinophil basic proteins (i.e. ECP, eosinophil peroxidase [EPO] or eosinophil-derived neurotoxin [EDN]) and stained more than 98% of peripheral blood eosinophils irrespective of their degree of activation. EG2 stained 15% of resting and 75% of activated eosinophils; EG1 recognized 74% and 78% of resting and activated cells, respectively. The numbers of BMK-13, EG1 or EG2-positive staining cells in bronchial biopsies from asthma were significantly greater than atopic non-asthmatics (P less than 0.02, P less than 0.01 and P less than 0.05, respectively) and normal non-atopic controls (P less than 0.001). For each of the various groups studied, the rank order for the number of eosinophils stained was BMK-13 greater than EG1 greater than EG2. BMK-13 stained significantly more cells from bronchial biopsies of atopic asthma and atopic non asthma when compared to EG2 (P less than 0.001 and P less than 0.05, respectively). Since only a proportion of BMK-13+ cells were EG2+, these results suggest that not all tissue eosinophils are actively secreting. Thus, BMK-13 can serve as a useful pan-eosinophil marker in tissue sections since it appears to stain most eosinophils.