By means of lucigenin-dependent chemiluminescence it has been demonstrated that films of organic polymers activate human neutrophils. The strongest response was always induced by polyvinyl chloride-D and the smallest by polyethylene. No significant differences were found between the number of adherent cells on the surfaces of the investigated materials. The stimulatory capacity of the organic polymers were changed after adsorption of some plasma proteins, These effects were not connected with similar alterations of cell attachment. A second stimulation by opsonized zymosan was only possible for neutrophils adherent to polyether urethane and polyvinyl chloride-D.