Several points indicate that extragalactic jets can interact with dense gaseous obstacles which occur on their ways. Examples of these interactions are the knotty structure of the radio and optical jet in M 87 and in other objects. These observations have been interpreted by Blandford & Königl in terms of collision of a jet with supernova remnants. We have reanalysed this idea taking into account new observations and improvements in the theory of diffusive shock acceleration. We find that the model requires a very high supernova birthrate (∼ 1 SN/year), which is not observed. It is more probable that the “obstacles” are formed by the stellar winds from the red giants. We estimate that the value of the magnetic field is ∼ 10−5 G in the interaction region (r=1kpc) (paper in preparation).