The report deals with the experiences of the author in respect of the structural analysis by means of X-rays of some molecules of medical and biochemical interest. In the first instance a general explanation is given of the development of structural X-ray analysis to its present-day importance. The systematic course of such a structural analysis has been demonstrated in its most important steps with penicillin and a few compounds of the vitamin B group as examples. 12 Resume L'expose traite des experiences de l'auteur dans la definition structurelle radioanalytique de quelques moIecules...