THE degree of blood supply in vital and non-vital tumour regions has long been of interest in tumour biology in its various aspects. In this paper evidence is presented that many tumours contain substantial regions which cannot readily be reached by blood-borne substances, and that these regions contain living cells capable of starting tumours when transplanted into new hosts. These regions were easily mapped out by changing the colour of the systemic blood with a harmless dye which, in addition, coloured the interstitial fluid, but did not enter the living cells (Goldacre, 1955, unpublished data; Goldacre and Sylven, 1959; Holmberg, 1961). Preliminary data (Goldacre and Sylven, 1959) indicated that in some tumours in rats and mice the only region presenting an open connection with the systemic circulation was a thin peripheral zone varying from a few millmetres to a tenth of a millimetre in thickness or less. Somewhat similar observations were made by Owen (1960) in spontaneous tumours of cats and dogs, using the same method. The blood, often visible in the regions unreached by the dye, was blocked off from the general circulation. In the present communication mor
