Serum biomarkers associated with Fasciola hepatica infection of Corriedale sheep were analysed during the first 12 weeks of infection using surface-enhanced laser desorption ionisation time of flight mass spectrometry (SELDI-TOF MS). In the discovery phase of analysis,pooled sera collected at week 0 and at each week p.i. to week 12 were fractionated by an ion-exchange chromatography and the protein mass fingerprints obtained in individual fractions were in the M/z range 1.5'150 kDa. A total of 2302 protein clusters (peaks)were identified that varied between time-points following infection with peaks increasing or decreasing in intensity, or showing transient variation in intensity, during the 12 weeks of parasite challenge. In the validation phase, candidate biomarkers in sera of individual sheep at weeks 3 and 9 p.i. were analysed, identifying 100 protein peaks, many of which are small peptides <10 kDa in size: 54% of these peaks were up-regulated in intensity at week 3 or 9 p.i. Twenty-six biomarkers were chosen for further study, ranging in size from 1832 to89,823 Da: six biomarkers were up-regulated at weeks 3 and 9 p.i., 16 biomarkers were up-regulated only at week 9 p.i. and four biomarkers were down-regulated at week 9 p.i. Two biomarkers up-regulated at week 9 were identified as transferrin (77.2 kDa) and ApolipoproteinA-IV (44.3 kDa), respectively. The results show that the interaction between the host and F. hepatica is complex, with changes in biomarker patterns beginning within 3 weeks of infection and either persisting to weeks 9'12 or showing transient changes during infection. Identification of biomarkers expressed during ovine fasciolosis may provide insights into mechanisms of pathogenesis and immunity to Fasciola and may assist in the rational development and delivery of vaccines.