Glucans are (1 3)-b-D-glucose polymers that are found in the cell wall of fungi, bacteria and plants. Glucans, isolated from natural sources, are known to stimulate humoral and cell-mediated immunity in humans and animals. In addition to the potent immune stimulatory effects of (1 3)-b-D-glucans, there are a number of toxicological effects associated with exposure to the water insoluble, micro-particulate form of the polymer. Recent investigations have also suggested a potential role for (1 3)-b-D-glucans in inhalational toxicity. Specifically, (1 3)-b-D-glucans have been implicated in the symptomatology associated with "sick building" syndrome. The mechanisms by which (1 3)-b-D-glucans mediate immune stimulation and, perhaps, toxicity are currently under investigation. It is now established that (1 3)-b-D-glucans are recognized by macrophages and, perhaps, neutrophils and NK cells via a (1 3)-b-D-glucan specific receptor. Following receptor binding, glucan modulates macrophage cytokine expression. Herein, we review the chemistry, immunobiology and toxicity of (1 3)-b-D-glucans and how it may relate to sick building syndrome.