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The isolation of bacteria-free explants, mostly buds or nodal segments as starting material for vegetative propagation in vitro is often problematic at the department. The bacteria in question are in general non pathogenic Pseudomonas, Flavobacterium, Alcaligenes and Escherichia spp. which do not seriously inhibit the growth of vigorous cultures. However, under less favourable conditions, e.g. during storage of germplasm, the bacteria may overgrow the cultures. Moreover, contaminated cultures are a menace to clean ones in the collection. Stimulated by the success of Phillips et ah, (1981) a number of antibiotics were tested on their effect on bacterial growth in contaminated cultures of Cinchona pubescens and a Cryptocoryne sp. Filter sterilised solutions of oxytetracycline, streptomycin, Chloromycetin (= chloroamphenicol), penicillin-G and rifampicin were added to autoclaved medium cooled down to c. 60°C, at concentrations of 10 and 50 mg/1. Rifampicin and the more stable gentamycin were, in concentrations of 10, 50 and 100 mg/1, autoclaved together with the medium for 20 min. at 115°C. To facilitate the observations on bacterial growth the crystal-clear gelling agent ‘Gelrite’ was used to solidify the medium.