It has been possible to study the cultural characters and certain of the immunological relationships of sixteen strains of Bacillus bovisepticus. The organisms have fallen into three distinct cultural groups. The largest group comprises eight strains, four of them obtained in pure cultures from cases of pneumonia in cows, two others were pure infections in calves, the others were associated with other organisms. This group may be characterized as short, non-motile, Gram-negative, encapsulated rods which fail to produce indole but produce clear zones about the deep colonies in horse blood agar plate cultures. Members of this group produce acid in media containing dextrose, lactose, saccharose, maltose, and mannitol. The final hydrogen ion concentration in dextrose broth lies between pH 6.3 and 6.1.

The next largest aggregation comprises six strains. These organisms ferment dextrose and saccharose, but do not attack lactose, maltose, or mannitol. All produce indole. All are soluble in ox and guinea pig bile but none are hemolytic.

The other two strains resemble the preceding but attack mannitol in addition to dextrose and saccharose. They produce indole but are not bile-soluble.

In the main the immunological relationship has been found to follow cultural grouping. All of Group I are agglutinated by a serum prepared by immunizing a rabbit with a single Group I strain. This serum fails to agglutinate members of other groups. The same holds true with Group II and III sera. In no instance has cross-agglutination between groups taken place.

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