Fifteen mice with Pasteurella pneumotropica orbital abscesses were noted in
mice that were homozygous for a targeted Cd28 gene mutation. Only one mous
e heterozygous for the Cd28 mutation was affected. According to phenotypic
reactions and 16S rDNA sequencing, the isolates were most similar to biotyp
e Heyl, This article provides evidence for an immunologic basis of suscepti
bility to P. pneumotropica infection.