The anther-idial branches produced on hyphae of A. ambisexualis E87 in response to hormone A were attracted by and grew to plastic particles on which hormone A had been absorbed. Antheridium-like structures were produced on the tips of these branches. It is suggested that hormone A may be the substance secreted by the oogonial initial that directs the growth of the antheridial branch to the initial and brings about the formation of the antheridium.