A crucial experiment is proposed to test the special relativity theory. It is based on the Mössbauer effect, and consists of mounting the source and absorber on two different rotors gyrating at the same speed, thus securing a relative speed between the two, while at the same time having both at the same absolute speed. It is able to distinguish between Einstein's and Lorentz' theories and should give a nonnull result if Einstein's theory is verified and a null result if Lorentz' is confirmed. This is an important result, since many authors consider Lorentz' theory as either invalid or as equivalent to Einstein's one. The experiment is feasible with current technology, is of low cost and unambiguous. © 1976 Società Italiana di Fisica.322519524