The in vitro antimicrobial activity of 2-amino-benzo[d]isothiazol-3-one and of several 2-arylideneamino derivatives carrying in the second position a substituted or unsubstituted aromatic ring or an arylalkenylidene moiety was determined by the broth dilution method against several strains selected to define their spectrum and potency. All the compounds demonstrated good antibacterial properties against Bacillus subtilis, streptococci, enterococci and staphylococci including penicillin-resistant clinical isolates. Several compounds showed excellent inhibitory properties against Streptococcus pyogenes, which is the most sensitive microorganism tested. Many benzisothiazolones exhibited good activity against Gram-negative Haemophilus influenzae. As regards antifungal activity, several of the tested compounds inhibited Saccharomyces cerevisiae at concentrations of 3-6 microg ml-1. In all cases the parent 2-amino-benzo[d]isothiazol-3-one was the most effective agent, with minimum inhibitory concentration (MIC) values ranging from 0.07 to 6 microg ml-1. The results obtained indicate that most of these compounds are wide-spectrum antimicrobial substances and promising agents against penicillin-resistant staphylococci.