Reference is first made to antibiotics and relates to the use of low, sub-therapeutic levels of such products given daily in the feed over long periods of time. EEC legislation has followed UK policy in restricting their use as growth enhancers almost exclusively to those of non-therapeutic use in man. In their role as growth enhancers their inclusion in the feed results in increases in liveweight gain and/or feed conversion efficiency and thus increase the efficiency of production. In the EEC it has been calculated that the benefit of using growth promoting antibiotics saved the equivalent at 1982 prices of 480 x 10 e.c.u. There would be some additional benefit from reduced incidence of disease in certain cases. Since the early 1950's, when antibiotics were first introduced as growth enhancers, to the present time there have been numerous scientific publications concerning their efficacy. Thus, from experiments involving many thousands of poultry and of pigs the following values have been given relating to percentage weight gain in poultry and pigs respectively 5.0 ± 0.6 and 6.5 ± 0.5; corresponding values for increase in feed conversion efficiency are 3.5 ± 0.4 and 4.1 ± 0.04; all four values quoted are percentage increases over controls. Relating to ruminants, use of monensin, an ionphore antibiotic at levels of 25-33mg kg-1 feed in 35 trials, resulted in increases over controls in weight gain and feed conversion efficiency of 5.2% and 8.65% respectively.