Four groups of inbred male LEW rats were examined: A, germfree athymic; B, specified pathogen free (SPF) athymic; C, germfree euthymic; D, SPF euthymic. All animals were killed at 18 weeks and compared with respect to body weight, histological appearance and cell density of the lymphoid organs, haematological values and differential counts of bone marrow, peripheral blood and lymph. Athymic rats had a lower body weight, less densely populated lymphoid organs, and fewer lymphocytes in the blood and lymph compared with euthymic animals. No difference was seen between athymic rats under germfree and SPF conditions, and in general the differences between athymic and euthymic animals were less pronounced under germfree conditions.