The responses of cattle infected with Fasciola hepatica to treatment with nitroxynil or closantel were monitored by faecal egg counts and by ELISA assay of anti-fluke antibodies. A first trial with experimentally infected heifers showed an increase in anti-fluke antibody titre as early as 2 weeks post-infection. Eggs were first detected in the faeces 10 weeks after infection. Egg output increased steadily over the next 8 weeks and then rapidly decreased. Treatment of a 20-week infection with nitroxynil was followed by a slow decrease in antibody titre 4 weeks later. This decrease continued over the next 40 weeks, but returned to pre-infection levels in only 2 out of 4 animals. The faecal egg count fell to zero 2 weeks after treatment and remained so for the following 30 weeks, although 1 animal produced a few eggs 32 and 34 weeks post-treatment. Within this period, neither diagnostic technique discriminated between this persistently infected animal and the others. In a second trial, 45 cattle from a naturally infected herd were treated with nitroxynil or closantel. The faecal egg counts of the treated cattle were zero within the following 2 months, whereas there were eggs in the faeces of the control (untreated) group. Nevertheless, the treated cattle showed a small, non-significant drop in anti-fluke antibody titre. These results demonstrate the need for new tools to monitor and evaluate accurately the efficacy of anthelmintic treatment.