Using computer image analysis we studied the development of dense cellular and dense lymphoid areas ("milky spots") and of pendant lymphatic nodules in mouse omenta after intraperitoneal immunization with sheep red blood cells. In both euthymic (BALB/c and hairless BFU) and athymic hairless nu/nu BALB/c mice the proportion of newly developing activated omental areas (AOA) was biphasic, with distinct peaks on days 3-4 and 8-12 after immunization, and a trough on days 5 and 14. There was a small difference between athymic and euthymic BALB/c mice. In comparison with the nu/nu BALB/c mouse, the BFU mutant had a lower proportion of AOA on days 4 and 10. The athymic state is not thought to have a great influence on the AOA development; this depends on a basic macrophage defence, which is well developed in the athymic model, and is self-regulated and shaped by a sequence of cell immigration, settling, differentiation and emigration.