Isolates of Branhamella catarrhalis from 13 patients with pneumonia, 6 patients with tracheobronchitis, and 8 patients who were colonized with the organism were studied with respect to susceptibility to the bactericidal action of normal human serum (NHS), glass slide hemagglutination (HA) of group O human erythrocytes, beta-lactamase production, and susceptibility to selected antimicrobial agents and laboratory drugs. A total of 18 of 27 isolates were serum resistant, 22 of 27 produced HA, and 21 of 27 were beta-lactamase positive. Statistically significant correlations were found between susceptibility to NHS and susceptibility to trypsin (r = +0.47; P = 0.01) and between susceptibility to NHS and HA (r = -0.48; P = 0.009). Significant correlations were also observed among several pairs of antimicrobial drugs. Analysis of variance showed that mean ampicillin MICs correlated with isolate group (r = -0.49; P = 0.03) in that the pneumonia isolates had higher MICs. Some phenotypic characteristics appeared to be independent of each other. These data suggest that important differences exist among clinically significant B. catarrhalis strains and that these differences may be due to differences in the cell wall envelope of the organism.