Substances comprising carboxylated and/or sulfated oligosaccharides in substantially purified form, including compositions containing same and methods of using same, are disclosed for the regulation of cytokine activity in a host. For instance, the secretion of Tumor Necrosis Factor Alpha (TNF-.alpha.) can be either inhibited or augmented selectively by administration to the host of effective amounts of substances or their compositions comprising specific oligosaccharides in substantially purified form. Thus, the present invention also relates to pharmaceutical compositions and their use for the prevention and/or treatment of pathological processes involving the induction of active cytokine secretion, such as TNF-.alpha.. The invention also relates to the initiation of a desirable immune system-related response by the host to the presence of activators, including pathogens. The substances and pharmaceutical compositions of the present invention may be daily, at very low effective doses, typically below 0.1 mg/kg human, or at intervals of up to about 5-8 days, preferably once a week.