On 31 farms, blood samples were taken from adult dairy cattle in September 1985, when pastured, and in November-December 1985, when stabled, to assess serum pepsinogen levels and level of nematode antibody titres. Faecal samples taken in September were examined to establish the presence of parasites by means of egg counts and larval identification. During the stabling period, dry cows were either treated with ivermectin or with a placebo in alternate sequence of expected calving date. As a result, 285 cows were treated with ivermectin while 242 cows served as controls. Anthelmintic treatment resulted in a significant increase in the 305-day milk production of 205.1 kg (P less than 0.01). Fat and protein percentages were not significantly influenced by anthelmintic treatment. There was a significant between-herd variation in nematode antibody titres and in pepsinogen values. The mean herd milk-production response to treatment correlated positively with the mean herd Ostertagia antibody titre measured in September 1985 (r = 0.364, P less than 0.05).