To clarify the origin of (1-->3)-beta-D-glucan in blood products and assess the biological activity of filter extracts, we evaluated (1-->3)-beta-D-glucan extraction from depth filters used to process blood products and their in vitro effects on proinflammatory cytokine production from macrophages. Cellulose or nylon filters were analyzed for (1-->3)-beta-D-glucan using the Fungitec G test. To evaluate the biological activity of the filter extracts, Mono Mac 6 cells (a human macrophage cell line) were cultured with filter extracts with or without lipopolysaccharide, and tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) concentrations in the culture media were measured. (1-->3)-beta-D-Glucan was released from seven cellulose filters but the nylon filter level was undetectable. Proinflammatory cytokine production ranged from 74.3% to 119.0% of the control for TNF-alpha and 81.2% to 115.9% for IL-1beta. TNF-alpha and IL-1beta levels were low without lipopolysaccharide. The data indicate that (1-->3)-beta-D-glucan in blood products is contaminated with the depth filters and that these filter extracts modulate proinflammatory cytokine production from macrophages.