Grafts made of two X-irradiated nucleated cell parts of Acetabularia mediterranea showed about the same dose-dependent reduction of the cap forming ability as previously found for ungrafted uninucleated cell parts. Similar grafts with one irradiated and one unirradiated partner showed no significant reduction of the cap forming ability. Twonucleated interspecies grafts between an irradiated partner and an unirradiated partner, i.e. A. mediterranea or crenulata, revealed both a dose-dependent and irreversible destruction of the species specific morphogenetic substances stored within the cytoplasm and of the nucleus-dependent production of these substances as judged by the morphology of the caps formed by these grafts.