The gene encoding an outer membrane lipoprotein (OmlA) was cloned from Actinobacillus pleuropneumoniae strain NG-8 (serotype 5a). The deduced amino acid sequence of OmlA from strain NG-8 showed 61% identity to the OmlA from serotype 1 strain, which confers protective immunity to pigs. Southern blot analysis showed the presence of a sequence highly homologous to the omlA gene of strain NG-8 in strains of serotype 5a, 5b and 10. A specific serum against OmlA of NG-8 also detected a homologous protein in the strains of these serotypes. These data shows the presence of antigenic variability among A. pleuropneumoniae OmlA proteins. © 1995 Academic Press, Inc.