Upon testing the individual fractions of a composite bacterial vaccine for biological activities, a potent immuno-stimulatory capacity could be demonstrated within a crude membrane proteoglycan preparation from Klebsiella pneumoniae. One of its characteristic features was the capacity to induce alpha-type interferon and increased NK activity in vivo in mice, following intraperitoneal or oral administration. A highly purified fraction from the crude preparation was obtained using alkaline hydrolysis, delipidation and size fractionation. This fraction was shown to be a very potent inducer of NK cells in vivo or in vitro, where in the latter systems concentrations as low as 0.1 microgramme per ml were highly efficient.