Abstract: A biologically active fraction of reduced toxicity or substantially free of it formed of components having molecular weights not exceeding 10,000-12,000, capable of stimulating in vivo the resistance to bacterial infections, is obtained from bacteria, more particularly Gram-negative bacteria.A process to that effect consists of subjecting a solution of an hydrosoluble crude bacterial extract to the action of an enzyme, such as DNase, which is capable of causing the irreversible detachment of the above said biologically active fraction from the hydrosoluble crude extract which can then be separated, such as by a filtration procedure, from the other components of the hydrosoluble crude extract. The latter can be obtained such as by the treatment of bacteria with a water-phenol mixture, preferably at a temperature above ambient, from about 60.degree. to about 70.degree. C., or by the action on bacteria of a detergent such as sodium dodecyl sulfate.