The efficacy of the morantel sustained release bolus was evaluated in 20 first-season grazing calves and 92 cattle in two separate trials. All animals grazed contaminated pastures and were exposed at the time of spring turnout to a risk of infection from gastrointestinal nematodes. In the first study 45 bolus-treated and 47 nontreated control (second or third season) cattle grazed similar but separate communal pastures, while in the second study nine bolus-treated and 11 nontreated control cattle grazed together on a single pasture. Efficacy determinations were conducted by frequent observations throughout the grazing season on faecal worm egg output, serum pepsinogen levels and liveweight gain. In the older group of animals (Trial 1), small differences were recorded between treated and control animals in faecal worm egg output and levels of serum pepsinogen as monitored throughout the grazing season. A significant mean liveweight advantage of 20.27 kg (P less than 0.001) was observed in the bolus-treated group. However, unrecorded pregnancy could not be excluded with certainty as a factor responsible for weight differences. The use of the morantel sustained release bolus provided an adequate protection against parasitic gastroenteritis in the first season grazing calves (Trial 2) despite the fact that bolus-treated animals were co-mingled with control animals. Both the worm egg counts and serum pepsinogen values were lower in the bolus-treated group. The morantel sustained release bolus was well tolerated in both trials.