Toner replenishing system

Developer container comprising: a hollow body; a shoulder portion (20a) contiguous to one end of said main body and constituting a wall; and a discharge mouth (23) contiguous to one end of said body and whose diameter is less than said shoulder portion; said discharge mouth including an annular collar (24) projecting outwardly, characterized in that on the outer periphery of said annular collar (24) there is provided a plurality of annular ribs (28) constituting a coupling portion to some protrusions or recesses, arranged on the inner periphery of a plug (29), which are complementary to said annular ribs (28).