Thymine in the acid-soluble fraction of Arbacia eggs. Mature Arbacia eggs were extracted with cold dilute perchloric acid, the extract concentrated, and the concentrate digested in hot perchloric acid. Thymine was recovered from the digest by paper chromatography, and the amount per egg found to be about 5 times the amount per sperm. This was the amount expected from previous experiments and is believed to represent all or almost all of the thymine in the egg. The result supports previous observations that DNA is absent from the mature egg although present in the nucleus of the egg in the germinal vesicle stage. No thymine could be recovered from a similar extract of 5,000 times as many sperm of the same species. The observations are consistent with the theory that DNA and its derivatives act as metabolic antagonists of the corresponding ribose compounds.