Lymphatic follicles in the spleen and lymph nodes of all animals were classified as primary and/or secondary follicles and as small, medium or large. The presence and degree of follicle depletion (none, mild, moderate or severe) was assessed on the basis of the cellularity of the germinal centres. Large, undepleted secondary follicles were interpreted as evidence of high activity, whereas small, depleted primary follicles indicated lowest activity. Similarly, bone marrow activity was classified as low, moderate or high based on the ratio of haematopoietic cells to adipose tissue in a cross section of the marrow in the sternum.