Various natural lignified materials and synthetic dehydrogenation polymers (the so-called synthetic lignins) prepared from several phenylpropenoids were investigated for their mitogenic activity. All of these materials time- and dose-dependently stimulated the DNA synthesis of isolated mouse splenocytes. However, their stimulating activity was significantly decreased when the splenocytes were pretreated with anti-Thy-1.2 monoclonal antibody and rabbit complement. Natural and synthetic lignins demand attention as potential immunopotentiators.