A valve is provided for mounting on the hub of an introducer needle to provide a flashback chamber, to prevent outflow of blood from the introducer needle and to allow a guide wire to be inserted into the introducer needle in a bloodless seal-tight manner. The valve employs a seal ring having a slit through which the guide wire may be passed. A port is also provided to vent air from the introducer needle through an air filter that prevents passage of blood.