Unpollinated tomato ovaries treated with certain synthetic growth substances can be stimulated to develop into seedless fruits, and the rate of development during the first six days is shown to be related to the amount of growth substance with which the ovary is treated. The details of this relationship have been worked out in a series of experiments in which ovaries were treated with known amounts of 2-naphthoxy-acetic acid (BNOA). With this as a basis, a quantitative test method has been evolved by which the activity of different growth substances and natural hormone extracts can be measured relative to that of BNOA. The method, as is described in the next article in this number (see p. 32), has been used to follow the changes in the hormone content of apple seeds throughout the season.