Tissue samples from all major organs from all animals were fixed in 4% buffered paraformaldehyde for 24-48 h prior to routine embedding in paraffin wax. Sections (5 mm) were stained with haematoxylin-eosin for histological evaluation. Sections from haemolymphatic tissues (spleen, pancreas of Aselli, bone marrow (sternum) and in selected cases mesenteric or mediastinal lymph nodes and thymus) were prepared for immunohistological examinations and the TUNEL method.