A microbiological survey of the effectiveness of class II (laminar flow) 'safety' cabinets found in the UK in the last six years is recorded. Only two of the nine units tested approached the containment of aerosols achieved by a good class I (exhaust protective) cabinet. The others were potentially hazardous if used with pathogenic material. The National Sanitation Foundation and the British Standards Institute have now laid down adequate specifications based on biological containment, and hopefully the conforming cabinets will be much better; even so, the purchase of a cabinet must be undertaken with care, and the cabinet requires frequent monitoring during use and after servicing.