Peritoneal cells of guinea pigs treated or not by intraperitoneal injection of minced polyester threads have been tested by the NBT test. In phagocytic cells nitroblue tetrazolium (NBT) is reduced to formazan. This is deposited as a dark blue precipitate in the cytoplasm. Changes in reduction capacity have been quantitated by calculation of percentage of macrophages containing formazan deposits, (Tab. 1), and by extraction and photometric evaluation of formazan (Tab. 2). Peritoneal cells incubated with polyester show an increased NBT reduction, which is more apparent with the photometric evaluation. A statistically significant difference in NBT reduction has been found between the peritoneal cells of the guinea pigs treated "in vivo" with polyester and those of the untreated "in vivo" with polyester and those of the untreated animals, whether or not challenged in vitro with polyester threads. These results suggest a change in cell population following polyester treatment.