The spatial distribution of viable carcinoma cells, necrosis, vascular volume, connective tissue, growth fraction, labelling index, mitotic index and grain count per cell nucleus was determined on autoradiograms of the Walker carcinoma 256 for tumours of different size and also as a function of distance from blood vessel. From the edge to the centre of the tumours, the portion of carcinoma cells, vascular volume and portion of connective tissue decrease whereas growth fraction and labelling index, at large, remain constant. From day 8 to day 12 after transplantation the percentage of carcinoma cells decreases from 65 to 24% and that of necrosis increases from 9 to 49%, whereas the frequency of stroma elements and the proliferative indices remain constant. With increasing distance from blood vessel all the parameters investigated show pronounced changes. The numbers of viable and of proliferating carcinoma cells decrease exponentially with distance from blood vessel and reach the 50% levels at distances of 100.2 ± 1.3 μm and 83.6 ± 3.4 μm (±S.D.), respectively. © 1979.