The usefulness of an antibiotic depends not only upon its antibacterial potency and spectrum but also on the prevalence of resistant organisms and the extent and severity of the adverse reactions to which it may give rise. Variations in formulation of the same compound are reflected in differences in bioavailability. These may be intentional, as in the development of long-acting preparations, but may also be unexpected following differences in drug purity, content and gastro-intestinal absorption. Individual and species differences in treated animals also result in variations in bioavailability. The merits of combination products of two or more antibiotics are often equivocal and some combinations are definitely disadvantageous. Practical rules for the selection and use of antibiotics emphasise the need for full doses of effective compounds used for limited periods with monitoring of patterns of sensitivity so that problems of large-scale resistance can be avoided.