The substances present in therapeutic penicillin, which cause inhibition of germination and root growth are represented by the indole-3-acetic acid type of compound. Since this and phenylacetic acid were known to be present in the penicillin tested, it is concluded that they are responsible for the inhibition activity against seeds. None of the crystaline penicillins tested appreciably retarded germination according to Walton J. Smith, Science #2705 p. 411.