Ca++ effects on the organization of human centrosomes isolated according to Bornens et al., were followed by double immunofluorescence technique. Ca++, at millimolar range, is able to modify the distribution of the pericentriolar material (PCM) and to decrease the intercentriolar distance. In the light of these results, we have slightly modified the centrosome isolation method and shown that centrosomes isolated in the absence of EDTA have several structural differences from the previously described structure. In particular, centriole diameter is decreased by a transverse sliding of microtubule triplets with respect to each other, suggesting the possibility of movements within centrioles themselves.