AbstractCleavage of fish (Oryzias latipes) eggs was induced by injection of heterologous (sea urchinHemicentrotus pulcherrimusand oysterCrassostrea gigas) sperm microtubules. Cleavage initiating (CI) activity of microtubules was higher in 3% PVP suspension than in 6% BSA, and not affected significantly by the concentration of microtubules themselves. The CI activity of microtubules suspended in 3% PVP was comparatively stable in the frozen state. Heat‐treatment at more than 55°C resulted in the loss of most or all of their CI activity. Such activity was observed in side‐by‐side aggregates of tubulin linear polymers of sea urchin (Hemicentrotus pulcherrimus) spermatozoa but not in dispersed linear polymers or tubulin dimers. Microtubules with CI activity seem to participate in initiating cleavage as astral centers, or a “seed” for polymerization of ooplasmic tubulins in acti