The production of heparinase by a wide range of anaerobic bacteria isolated from clinical specimens was investigated. None of the 29 strains of Bacteroides fragilis produced heparinase. Of 62 other Bacteroides tested, only two of four strains of B ovatus, two of three strains of B thetaiotaomicron, and two of four strains of B uniformis were heparinase producers. None of the 48 strains of fusobacteria or seven strains of Veillonella produced heparinase. The anaerobic cocci (19 peptococci and seven peptostreptococci) were also negative for heparinase production as were 46 Clostridium spp tested. It was concluded that heparinase production by anaerobic bacteria was unlikely to play a part in the regional thrombophlebitis that sometimes occurs in anaerobic infections.