THERE is considerable evidence that the cytoplasm of the cell contains structures capable of carrying out protein synthesis1, and it has been shown in one case2 that formation of protein may be little affected by removal of the nucleus. These findings seem to exclude the appealing hypothesis that the nucleus is the source of the cell's proteins. It does not follow, however, that the nucleus is incapable of protein synthesis; and in fact there has been some evidence3 that proteins may be formed in the nucleus. The results described below seem to demonstrate unequivocally the existence of protein synthetic mechanisms that are associated with the nucleus.