A special collection of 336 Neisseria meningitidis strains was established that spanned the genetic variability, as defined by multilocus enzyme electrophoresis, of the ET-37 complex (228 strains isolated in different continents between the 1960s and the 1980s) and of other serogroup C meningococci (108 strains). Of the strains in the ET-37 complex, 90% were serogroup C and 10% were serogroup B. Most ET-37 complex strains were serotype 2a and serosubtype P1.5,2 or P1.5,y; most expressed class IIb pilin. Twenty-six Opa proteins differing in electrophoretic mobility or reactivity with monoclonal antibodies were variably expressed by different members of the ET-37 complex, although only four opa genes were detected in individual strains. Despite this overall diversity, most isolates from any one outbreak were fairly homogeneous.