<正> The changes of thymic structure and functions of Ehrlich ascitescarcinoma-bearing mice were observed by using histometer methods

scanning electron microscopy

cell electrophoretic technique

and footpadtest etc.The results show that the changes are different at differentstages of tumor development.At the early stage

the synthesis of DNAand RNA and lymphoid cell mitosis in host thymus increased mar-kedly

while the thymic weight and thymocyte number decreased.It is suggested that more thymocytes are produced at this stage

butthe newly produced thymocytes

being stimulated by tumor antigensor other factors

are rapidly exported to peripheral lymphoid organsand blood

where they may be mobilized to participate in antitumorimmune responses.At the middle and late stages

the synthesis ofDNA and RNA

mitotic index and number of thymocytes in thehost thymus decreased and the thymus atrophied progressively.Theepithelial reticular cells contracted and gathered to form“epithelialislets”.Some of them were degenerated.These results indicate thatmuch fewer thymocytes are produced at these stages because of thedisintegration of thymic microenvironment.This event results inthe decrease of T lymphocyte migration from thymus to peripheralcirculating pool.Two thymus-dependent immune responses weredecreased during tumor development

but the mechanism is different.The insufficiency of thymus-dependent humoral immunity is dueto the decrease of helper cell activity

while the defect of cell-mediated immunity is due to the increase of suppressor cell activity.The possible mechanism and signifficance of structural and functionalchanges in the thymus of tumor-bearing hosts are discussed.