Antheridia were induced by exogenously applied GA3 at concentrations between 10-6 and 3 × 10-4 M in very young filamentous protonemata of Lygodium japonicum grown in darkness; the longer the dark preculture of protonemata, the lower was the sensitivity of the protonemata to GA3 . Antheridial initials were discernible after 36 hr of GA3 treatment in the most sensitive protonemata, and the timing of antheridial initiation was delayed with increasing protonemal age. This quantitative response of the protonemata provided the basis for a new method of assaying gibberellins in terms of the degree of antheridial formation. According to this method, all the gibberellins tested and one of their precursors were active in inducing antheridia in the protonemata, and the activity spectrum of the gibberellins was as follows: GA7 >GA4 >GA9 >GA3 >GA5 >GA1 >GA8 . The amounts of antheridiogen contained in conditioned media were measured by the present bioassay. A semi-logarithmic relation was shown between the percentage of antheridial formation and the concentration of conditioned medium within a certain dilution range. The amounts of antheridiogen secreted by the prothallia were quantitatively compared by transferring samples onto fresh media for a short period of time.