Forty glucosamine derivatives were tested for antimicrobial properties. Five of these were shown to possess significant activity against a number of bacteria, yeasts, and fungi in vitro. These were 5-bromosalicylidene-D-glucosamine (No. 103), cinnamylidene-D-glucosamine (No. 112), 3,5-dibromosalicylidene-D-glucosamine (No. 104), 3,5-dibromosalicylidene-D-glucosamine oxime (No. 130), and 3-nitro-5-chlorosalicylidene-D-glucosamine (No. 119). The acute LD50of each compound in mice was approximately 200, 400, 400, 600, and > 1000 mg/kg for 119, 104, 130, 103, and 112, respectively. Studies on mode of action indicated that 103, 104, and 130 may interfere with glucosamine metabolism inCandida albicans; ultrastructural changes in this organism were also produced by these three derivatives. Numbers 103, 104, 119, and 112 were phosphorylated by yeast hexokinase, and all except 112 depressed the rate of glycolysis in a cell-free extract.