We have examined the toxicity of over twenty antibiotics to protoplast-derived cells of Nicotiana plumbaginifolia. The least toxic antibiotics are the betalactams: ampicillin, carbenicillin and the cephalosporins can be used to provide broad spectrum antimicrobial activity without significant toxicity to plant cells. Similar broad spectrum activity can also be obtained by combining rifampicin and trimethoprim. Other antibiotics which may be useful are erythromycin and colistin. The aminoglycosides are not recommended.