Feed additives are widely used in beef production systems. Two trials were made to measure the effect of feed additives on the daily gain and feed efficiency of fattening beef cattle. In the first trial 48 Friesians and 48 weaned suckled calves weighing about 390 kg were fed individually on grass silage ad libitum together with 4 kg (Friesians) and 3 kg (weaned calves) bruised barley daily. The treatments were control (C), no additive; Avotan 150 mg/d (A); Flavomycin 45 mg/d (F) and Romensin 200 mg/d (R). Minerals and vitamins were given daily on the silage. In the second trial there were 240 Friesian and Exotic x Friesian bulls. They were housed on slats in groups of 15 and offered a mixture of bruised barley and soya bean meal ad libitum. The treatments were control, no additive; Flavomycin 50 mg/d, Romensin 210 mg/d and Salinomycin 150 mg/d. In trial 1 the steers were implanted with Ralgro and the heifers with Finaplix. No implants were used in the second trial. All the animals were weighed regularly and feed intake was recorded continuously. They were slaughtered when they achieved the required conformation and fat cover.