The present invention discloses the existence of two novel proteins UspA1 and UspA2, and their respective genes uspA1 and uspA2. Each protein encompasses a region that is conserved between the two proteins and comprises an epitope that is recognized by the MAb 17C7. One or more than one of these species may aggregate to form the very high molecular weight form (i.e. greater than 200 kDa) of the UspA antigen. Compositions and both diagnostic and therapeutic methods for the treatment and study of M. catarrhalis are disclosed.