Three groups of four calves were experimentally infected with infectious larvae of Mecistocirrus digitatus. One group received a trickle infection of 5000 L3 per day for 8 days, the other two groups received a single infection of 5000 and 40,000 L3, respectively. All animals were necropsied 120 days after infection. Prepatent periods varied between 61 and 79 days, and maximal faecal egg output was reached between 80 and 100 days after infection. Repeated infections were not additive and worm counts at 120 days after infection varied between 100 and 440 in the first two groups and between 120 and 1700 in the last group. There was no significant difference in worm counts between the different infection regimes. However, there was a significant positive relationship between worm burden and faecal egg counts. In addition, there were significant negative relationships between worm burden and packed cell volume, and weight gain. The relative decreases in packed cell volume and weight gain emerged 70-80 days after infection. Serum pepsinogen levels were significantly elevated by the end of the trial, but the observed positive relationship between worm burden and pepsinogen was not significant. An enzyme immunoassay based on crude adult antigen was able to detect M. digitatus infection at 90 and 100 days after infection, but again there was no significant association between worm burden and antibody levels. Therefore, anaemia and a reduction in weight gain caused by the haematophagous activity of adult stages seem to be the most important pathogenic effects of M. digitatus infection in calves.