The antimicrobial activities of crude dichloromethane fractions from acetone extracts of Agrobacterium rhizogenes transformed roots and roots of field-grown plants of Salvia sclarea as well as four pure abietane diterpenoids isolated from the hairy root cultures were determined. The growth of Gram-positive bacteria (Staphylococcus aureus, S. epidermidis, Enterococcus faecalis) but not Gram-negative ones (Escherichia coli, Pseudomonas aeruginosa) or pathogenic fungi (Candida albicans) was inhibited by fractions tested at concentrations of 37.5-75.0 microgml(-1). Abietane diterpenoids: salvipisone, aethiopinone, 1-oxoaethiopinone and ferruginol were shown to be bacteriostatic as well as bacteriocidal for the cultures of S. aureus and S. epidermidis strains, regardless of their antibiotic susceptibility profile. This was demonstrated by using simultaneously the optical density measuring method and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide-reduction assay. The highest activity was shown by salvipisone which demonstrated also a very interesting activity when its effect on 24-h-old staphylococcal biofilm cells viability was examined. It limited the survival of biofilms formed by S. aureus as well as by S. epidermidis, putting this compound to the list of potential anti-biofilm agents, better than most of known antibiotics.