Unit (6) for infusion of a replacement solution with a dialysis machine； said unit (6) including a washing device (7) that internally defines a washing chamber (9a) and which includes a connection cone (17) that extends inside the washing chamber (9a) and suitable for hydraulically connecting to a replacement solution line inside a dialysis machine； a connection device (1) suitable for connecting said connection cone (17) to a conduit to infuse the replacement solution to a patient； and a sealing element (8) suitable for closing the washing device (7) to isolate said washing chamber (9a) if the unit (6) is in a washing conformation； wherein said connecting device (1) includes a tubular element (2) having a first connecting end (2a) suitable for connecting to said connecting cone (17) of said washing device (7) and a second joining end (2b) suitable for connecting to an infusion line； and an annular locking nut (3) housed with freedom of rotation around the tubular element (2) in which it is axially retained and structured to engage and lock said washing device (7) if the connection unit (6) is in a dialysis conformation； said annular locking nut (3) including a handle (4) and a threaded portion (5) suitable for engaging a threaded portion (12) of said washing device (7).Unidad (6) para la infusión de una solución de sustitución por una máquina de diálisis； incluyendo dicha unidad (6) un dispositivo de lavado (7) que define internamente una cámara de lavado (9a) y que incluye un cono de conexión (17) que se extiende dentro de la cámara de lavado (9a) y adecuado para conectarse hidráulicamente a una línea de solución de sustitución dentro de una máquina de diálisis； un dispositivo de conexión (1) adecuado para conectar dicho cono de conexión (17) a un conducto para infundir la solución de sustitución a un paciente； y un elemento obturador (8) adecuado para cerrar el dispositivo de lavado (7) para aislar dicha cámara de lavado (9a) si la unidad (6) está en una conforma
