In the commonest causes of impaired renal transplant function there is reduced renal and glomerular perfusion. We investigated the possibility of identifying this phenomenon in renal transplant biopsies, by the crude but simple method of counting the mean numbers of glomerular capillary loops which contained erythrocytes. In cyclosporin toxicity, acute tubular necrosis and in acute cellular rejection there was a considerable reduction in the numbers of erythrocyte-containing glomerular capillaries. The separation of the acute tubular necrosis group from normal was almost complete. This feature is therefore not specific but appears to correlate well with states of impaired glomerular perfusion. We suggest that a simple assessment of glomerular erythrocyte 'load' forms a useful part of the histological examination of renal transplant biopsies.