Four groups, each of six male Friesian calves, were set-stocked on separate 0.66 ha paddocks from May 7 until October 23 1986. Each of the animals in groups 1 and 4 was dosed with an oxfendazole pulse release bolus at turn out whereas the animals in groups 2 and 3 were left untreated. Parasite-free naive tracer calves were introduced into each paddock for a limited period 12 days after turn out and again at the end of the trial. No adverse reactions or clinical signs were observed in either of the groups of calves which received boluses. The development of clinical parasitic gastroenteritis in both the untreated groups necessitated the humane slaughter of two animals and emergency anthelmintic treatment of the remainder. The lower plasma pepsinogen concentrations, and lower faecal egg and larval counts and worm burdens post mortem, together with the absence of clinical signs of parasitic gastroenteritis and bronchitis in the treated calves, confirmed the high efficacy of the bolus treatment.