Washing of the supporting frames of units for transporting live poultry from the breeding farm to the slaughterhouse is carried out, following upon extraction of the drawer-type cages containing the poultry, after positioning the supporting frame in a condition where it is rotated through 90° with respect to the transport configuration. Washing is carried out via a network of tubes and spray nozzles that can be displaced vertically from a raised, inoperative, position to a lowered, washing, position, where said network sets itself around the rotated supporting frame. After the supporting frame has been washed, it is repositioned in the transport configuration.