Abstract: AT-581, o-bis-(2-chloroethyl)-aminomethyl-phenylalanine dihydrochloride or ocaphane, is a new antitumour agent synthesized at our Institute. It has a strong inhibitory action on a broad spectrum of experimental tumours. It has also been shown to be effective against some human cancers. In the present paper the combined action of AT-581 and surgery on the Brown-Pearce carcinoma of the rabbit was investigated. 1. When the Brown-Pearce carcinoma cells were inoculated into the testes of rabbits, AT-581 produced an inhibiting effect on the metastases of the turnout into the livers (24—41%), the lungs (27—33%), and the kidneys (28—30%). The complete extirpation of the original tumour foci had no such action, whilst the combined therapy of AT-581 and operation significantly increased the therapeutic effect on the tumout metastases to the livers and lungs. 2. The effectiveness of the combined therapy varied according to the time of drug administration. It was found that one week's drug administration during the period including pre-and post-operative stages was adequate. 3. AT-581, operation, and the combined therapy have no obvious influence on the haemoglobin content and red blood cell counts, but a slight diminution of white blood cell counts was observed during the combination treatment.