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Abstract The antimicrobial activities of twenty antibiotics belonging to different chemical classea were detei’oiined in thepresefla ot the vitaminsi B1 E2, B6, nicotinamide, p—amicbonzoio acid ahd Ca The act1ities of the antibiotics belonging to the aminog1yoside, niacrlido and polyone groups, in general, were moderately affected by most vitamins. The ponicillins, cepholosporins, polymyxin and were the least sensitive to the antagonstic effect of the vitamins. Lincomycin, nystatin and the totraoyclincs suffered the greatest losses in the presence of vitamin B2. Maximum inactivation of nystatin oc’urred at 0.1 % vitamin B2 concentration • This inactivation was shown by spoctrophotometric, chromatographic and microbio1ogical techniques to be the result of chemical doi3radtion of the antibiotic. The degradation product wa devoid of antifungal activity. This degradation was found to be quite rapid, since it was complete in about 15 minutes. It was slightly potontiatod by vitamin C while it was unaffected by the presence of vitamins B, B6, and nicotinamido. A photo—oxidation reaction was most probably involved. |