In this study, M. catarrhalis was isolated from 68 of 200 (34.0%) sputum samples of patientsrnwith community-acquired pneumonia. It was also found in 56 (28.0%) of the nasopharynx ofrnthese patients. There were 42 (61.8%) males. and 26 (38.2%) females with pneumonia causedrnby M. catarrhalis. Fifty one of the 68 (75.0%) had chronic underlying diseases. Of the isolatesrnfrom sputum 37 (54.4%) and 32 (57.1%) of the 56 nasopharyngeal isolates were p-Iactamasernpositive. An ELISA was used to determine the antibody response of sera of patients withrnpneumonia caused by M. catarrhalis to the outer membrane proteins (OMPs) of the bacteriumrnfor both IgG-A-M and IgG31eveis in the sera. It was found that 40 of the 68 (58.8%) patientsrnwith pneumonia caused by this organism showed significant rise in the total immunoglobulinrn(IgG-A-M) level, and 43 of 68 (63.2%) in the IgG3 level in their convalescent sera. In therncontrol sera only I of 30 (3.3%) patients had significant rise for IgG-A-M while none of themrnshowed significant increase in titre for IgG3 (P< 0.05). Using lipopolysaccharide (LPS)rnantigens, only 16 of 68 (23.5%) paired sera showed significant increase in antibody titre forrnboth IgG-A-M and IgG3 compared to 2 of 30 (6.7%) of controls (P< 0.05). Significantrnbactericidal activity of the convalescent-phase sera at a dilution of 60% was detected in l3rnof 15 (86.7%) of the sera assayed. The same sera without complement were used as controlsrnand none of the controls showed significant bactericidal activities (P< 0.05).