SummaryBacillus larvaeproduces, in culture and in the host, a complex of proteases, some of which have been reported to be toxic for the host. The proteases were separated by gel filtration into two main peaks of different molecular size, and also separated from products of tissue breakdown characterized by their odour, fluorescence, and colour. The relatively large molecular sizes of the proteases may explain their apparent absence in fluids from young rapidly growing cultures ofB. larvae.The odour was separated, by solvent extraction, into a water-soluble sulphhydryl-containing material with an odour common to many bacterial cultures, and a strongly smelling ether-soluble substance, possibly phenylacetic acid, arising specifically from breakdown of host tissues.