Erythrocytes offer the exciting opportunity of being used as carriers of therapeutic agents. Encapsulation within erythrocytes will give the therapeutic agent a clearance equivalent to the normal life of the erythrocyte therefore maintaining therapeutic blood levels over prolonged periods and also giving a sustained delivery to the monocyte-macrophage system (reticulo-endothelial system). Both the dose and frequency of therapeutic interventions could thus be reduced. Ensuring a near-physiological survival time of carrier erythrocytes is essential to their successful use as a sustained drug delivery system, and this has not been demonstrated in man. In this study we assessed the survival in vivo of autologous unloaded energy-replete carrier erythrocytes in nine volunteers, using a standard 51Cr erythrocyte-labelling technique. Within 144 h after infusion there was a 3 to 49% fall in circulating labelled cells, followed thereafter by an almost complete return to initial circulating levels; surface counting demonstrated an initial sequestration of erythrocytes by the spleen and subsequent release. Mean cell life and cell half-life of the carrier erythrocytes were within the normal range of 89 to 131 days and 19 to 29 days respectively. These results demonstrate the viability of carrier erythrocytes as a sustained drug delivery system.