The postweaning response of beef calves to different levels of gastrointestinal parasite larvae (L₃) ingestion was investigated. Newly weaned calves (n = 77, 6 months old) were trickle-dosed with 0, 1000, 2000, 4000, 7000 or 10 000 L₃ (75% Cooperia oncophora, 10% Ostertagia ostertagi, 15% Trichostrongylus axei) per day for 10 weeks or drenched with anthelmintic at 3-weekly intervals. Liveweight, faecal egg count and blood serum pepsinogen were monitored regularly. Three calves per treatment were slaughtered following infection to determine adult worm burdens. For the following 11 weeks, liveweight, faecal egg count and serum pepsinogen were monitored for the remaining calves. During the dosing period, calves that were drenched had similar liveweight gain to those receiving up to 2000 parasites per day but grew 0.13–0.20 kg per day more than calves receiving between 4000 and 10 000 L₃ per day. Faecal egg counts did not reflect liveweight gains or dose rate for calves receiving more than 2000 L₃ per day and pepsinogen concentration did not reflect liveweight performance. Abomasal worm burdens at the end of dosing were highest in treatments receiving 4000 or more L₃ per day but intestinal (Cooperia species) burdens were highest in those receiving low infection rates. Liveweight gain during the carryover period for the 4000 L₃ treatment was significantly lower than in other treatments. Calves receiving anthelmintic showed a 14–40 kg liveweight advantage by the end of the experiment. This study indicates that to avoid production losses from parasites in beef calves in the absence of anthelmintic the ingestion of parasite larvae should be below 4000 per day.