A male-female adapter (21) can be coupled to a hermetically closable connector (1) with external threading (13) and closing plug (6) which can be bored by pressure of a male fitting so as to form a connector (1)—adapter (21) unit which is suitable for using a female fitting. The adapter (21) comprises an external tubular body (22) with an axial extension (24) which can be hooked to the connector (1) in a rotatable and axially slidable manner, an internal tubular body (26) with terminal tangs (27, 37) axially projecting from the extremities of the external tubular body (22), a middle transversal wall (28) which integrally connects the two tubular bodies (22, 26), and two opposite sense threadings (29, 30) formed in internal portions of the external tubular body (22), which threadings are separated by said middle transversal wall (28).