Serum IgE responses during primary or challenge infections with Trichostrongylus colubriformis were examined by measuring total IgE and parasite-specific IgE by ELISA. Three- to four-month-old lambs reared nematode-free received a single primary infection of 30,000 T. colubriformis-infective larvae (TcL3). Infections were terminated after 100 days by anthelmintic treatment, at which time faecal egg counts had fallen to low levels. Total serum IgE increased slowly from 20 days p.i. until anthelmintic treatment. The specific IgE response to T. colubriformis adult antigen peaked at 20-27 days p.i. before returning to near baseline levels. A further slight increase occurred between 56 and 100 days p.i. The animals were then divided equally into two groups. A first series of challenge infections consisting of either weekly 10,000 TcL3 infections (Group A) or a single 30,000 TcL3 infection (Group B) produced low faecal egg counts. Total and specific IgE to adult and L3-ES antigens increased rapidly over a 21-day period, then remained elevated irrespective of challenge regime. Termination of primary and the first challenge infections with anthelmintic resulted in a rapid decline in serum IgE responses but not other T. colubriformis-specific Ig responses. A second series of challenge infections consisting of repeated 10,000 (Group A) or 20,000 TcL3 infections (Group B) resulted in all IgE responses peaking within 7-8 days p.i. Marked softening of faeces coincided with peaks of specific and total IgE responses during challenge infections. The results of this study showed that infection of sheep with T. colubriformis leads to elevated levels of total and parasite-specific IgE in serum. IgE responses following challenge suggest involvement of this antibody isotype in protection from T. colubriformis infections.