AbstractStudies on serum/blood samples from European otters (Lutra lutra), a species under protection in most countries, were undertaken. Building of a two‐dimensional electrophoresis (2‐DE) protein identification map was started, identifying serum proteins by pattern comparison with cat or human serum protein maps and by immunoblotting using cross‐reacting antibodies against the respective human serum proteins. The capture of endangered species is very restricted, thus making fresh samples almost unavailable; most investigations therefore had to be performed on samples collected from animals found dead. Patterns obtained from specimens of this source were compared to the established pattern of serum proteins and of blood or liver cells. Some of the major differences observed can be explained by contamination with cellular proteins and/or the onset of lytic processes generating breakdown products, as shown by preliminary in vitro experiments. Better knowledge of “normal” serum protein patterns and interfering products will facilitate interpretation of data collected from samples with a less defined history.