Senescing tobacco leaf disks were treated with hydroxyproline (hypro). Chlorophyll breakdown and α-amino-nitrogen increase were partially inhibited, but the decrease in soluble protein was stimulated. The normal increase in absolute acid-phosphatase activity was reversed, but the increase in peroxidase activity occurred although after a lag phase. Kinetin inhibition of senescence was reversed by hypro and kinetin actually reinforced the effects of hypro. Proline only partially overcame the effects of hypro plus kinetin. In the presence of kinetin, proline considerably stimulated the increase in peroxidase activity.Discussed are the way in which hypro may be acting, and the possibility of peroxidase being involved in proline hydroxylation.