Summary Alginates are polysaccharides with gel-forming properties composed of 1,4-linked β-D-mannuronic acid (M), α-L-guluronic acid (G), and alternating (MG) blocks. Alginate can be used as a matrix for implanted cells in vivo. In this study, we have examined the ability of alginates and their components to stimulate human monocytes to produce tumor necrosis factor-α, interleukin-6, and interleukin-1. Alginates stimulated the monocytes to produce high levels of all three cytokines. Low G alginates were approximately 10 times more potent in inducing cytokine production compared with high G alginates. The M-blocks and the MG-blocks, but not the G-blocks, stimulated the cytokine production. The results demonstrate that the mannuronic acid residues are the active cytokine inducers in alginates.