Lignin is a heterogenous natural product composed of phenylpropane units and is usually associated with hemicellulose in its native state. Until now little attention has been paid to the potential therapeutic utility of lignified products. Natural lignified products are demonstrated in the present study to stimulate iodination significantly (incorporation of radioactive iodine into an acid-insoluble fraction) of human peripheral blood polymorphonuclear cells (PMN). This stimulation was significantly inhibited in the presence of myeloperoxidase inhibitors. These materials were almost completely deprived of their stimulation capacity by treatment with NaCIO2, but this capacity was not affected by severe treatment with H2SO4 or trifluoroacetic acid. Similar stimulating activity by chemically defined tannin-related polyphenolic compounds was observed. Degradation products or component units of lignin, and natural antitumor polysaccharides and their chemically modified derivatives (introduced with negatively or positively charged groups) and polysialoglycoproteins had little or no activity. The results indicate the importance of a polymerized phenolic structure for the stimulation of PMN iodination. Possible physiological relevance of the stimulation of iodination by lignified substances is discussed.