A 16-year-old girl who was treated with ceftriaxone developed two intravascular haemolytic attacks that led to acute renal failure and death. The direct antiglobulin test was positive only for C3d, and no ceftriaxone-dependent antibodies were detectable. Her serum reacted strongly positive with red blood cells in the presence of ex vivo antigen related to ceftriaxone (urine samples from patients receiving the drug). This is the first reported case in which the causative antibodies appeared to be stimulated solely by a degradation product of ceftriaxone.