Feed additives are widely used in beef production systems. Two series of trials were carried out during a 2 year period on several farms in the N.E. of Scotland to determine the effects of the feed additives, Salinomycin, Flavomycin, Romensin and Avotan on the growth rate of beef cattle. In the first trial which was carried out on 27 farms, there were 1650 steers and heifers which were on an ad-libitum diet of grass silage and a rolled barley supplement. The additives were pre-mixed with 1 kg ground barley and fed once daily to the groups of cattle as part of their concentrate allowance. The additives compared were Salinomycin 150, Flavomycin 40, Romensin 200 and Avotan 150 mg/head/day. All cattle were weighed at 0, 42, 84 and 126 days. The responses in terms of daily gain and the persistency of those responses were measured for the various feed additives. In the second trial which was carried out on 13 farms, there were 470 bulls which were in cereal-beef units. The additives used were premixed and fed in a similar manner to that in trial 1. The same additives were compared at similar dose rates to the other trials. All cattle were weighed at 28-42 day intervals and daily gains were recorded. On one farm, feed intake was recorded for each group of bulls given the different feed additives. The data from each farm was analysed using the analysis of variance technique. The mean gains on each farm were subjected to further analysis using least squares techniques to provide overall mean values with appropriate standard errors.