The gross protease activity of pathological samples of American foulbrood-infected cadavers from several UK sources was studied. In all cases the bulk of the activity is caused by neutral protease(s) (optimum pH ca. 6.8) that are inhibited by chelating agents such as EDTA and 1,10 phenanthroline (indicating metalloproteases) but not by inhibitors of other classes of proteolytic enzymes. The proteases, which derive from the infectious agent of AFB, Paenibacillus larvae, were unusual in being insensitive to phosphoramidon and in not degrading FAGLA, the artificial substrate specific for most Bacillus metalloproteases. The enzymes in AFB ropes and scales had temperature optima of 60-65°C and were inactivated quickly on incubation at 80°C. Activity at moderate temperatures (37°C) was great on general substrates such as casein, gelatin, and hide powder azure, slight on elastin-Congo red, and nonexistent on collagen. In SDS-polyacrylamide gels the enzymes from the various sources all had molecular weights about 24 kDa. The proteases could be detected only zymographically after brief washing to remove SDS. On silver-stained gels no bands corresponding to the enzymes' activities could be detected. On native polyacrylamide gels enzyme activity was resolved zymographically as at least three metalloprotease bands with samples from different sources showing a variety of patterns.