Objectives To localise the immunogenic part of infliximab and evaluate the clinical usefulness of measuring antibodies against infliximab fragments. Design Observational study. Settings A specialised inflammatory bowel disease (IBD) centre in a tertiary hospital. Interventions Serum was collected from patients with IBD and controls. Antibodies against whole infliximab (ATI) and against the digested Fc, F(ab′) 2 and F(ab′) fragments were measured by a specifically developed ELISA and by western blotting. A separate ELISA was used to determine infliximab levels in serum. Results 109 serum samples from 62 infliximab-treated patients were tested along with 64 control samples. Anti-F(ab′) 2 antibodies were found in 28/42 (67%) samples with positive ATI, all from infliximab-exposed patients. Anti-F(ab′) 2 antibodies were also present in 26 of the remaining 67 (39%) samples from exposed patients despite absent ATI. No specific anti-Fc antibodies were detected. Low trough infliximab level and high ATI level was found in 10/12 patients (83%) with complete loss of response to infliximab, but in only 5/14 patients (36%, p=0.02) who regained response to intensified infliximab regimen and in 2/24 patients (8%, p 2 antibodies showed similar test characteristics to ATI in patients losing response to infliximab, they were also detected in 61% of patients in maintained remission, thereby limiting their clinical usefulness. No cross reactivity to adalimumab was noted. Conclusions F(ab′) 2 is the immunogenic fragment of infliximab. However, ATI level in serum—combined with measurement of trough infliximab level—is better correlated with the clinical response to infliximab or with its loss.