Abstract: A method for measuring the mean velocity (VH) of an extracorporeal blood fluid or infusion fluid, by means of retroinjection interferometry, comprises the step of emitting a first laser light beam (41), from the laser cavity (40) of a semiconductor laser source (60), reflection of a second laser beam (45) by the fluid (50) and consequent generation of interference with the first laser beam (41) inside the laser cavity (40), detection of the interference signal by a monitoring photodiode (46), and processing, by means of an electronic processing and control circuit (100), of the interference signal detected. The invention also comprises an apparatus (62) for implementing the method described and an extracorporeal circuit (58) comprising said apparatus. The invention also comprises a method for replacing a laser source in said apparatus.