In Paracentrotus lividus embryos, a Mediterranean sea urchin species, HSP70 is present in all the cells. During cell division it localizes under normal growth conditions on the centrosomes and on the whole isolated mitotic apparatus. Now, in situ hybridization, Western blot analyses, and immunohistochemistry show that the HSP70 mRNA is present in both small and large P. lividus oocytes, that all four isoforms of HSP70 can be found also in the oocytes, and that a certain amount of HSP70 localizes on asters and spindles during polar body formation. Moreover, two representative cell-cycle related proteins, cyclin B, and Cdc2, are present both in small and large oocytes, concentrating in the germinal vesicles before its breaking down. Cdc2 has been found in the cytoplasm of small oocytes and in the germinal vesicles of the large ones and then together with HSP70 on the mitotic apparatus of the dividing oocytes. (C) 2003 Elsevier Inc. All rights reserved.