It has been shown by others that treatment of red cells with very small amounts of tin in the form of stannous citrate or stannous pyrophosphate before incubation with 99mTc results in the subsequent binding of almost all the added technetium and that when the labelled cells are injected into the circulation the rate of elution of 99mTc is less than when cells are labelled by first adding 99mTc, then tin. Because of the instability of stannous salts in solution, methods have previously been described for the freeze-drying of very small aliquots of tin in ‘kits’, specially designed for the labelling of red cells with 99mTc. The present paper confirms the advantage of‘pre-tinning’in labelling red cells with 99mTc and describes a method for which no special kit is needed and in which even smaller amounts of tin than have been used previously give optimal results. A dilution of stannous chloride which provides the requisite very low concentration of tin is prepared by a simple two-stage procedure. Since uptake of 99mTc is about 95%, only a single wash is necessary after labelling. Estimates of red cell volume based on a single sample at 10 or 20 min after an injection of labelled cells, uncorrected for loss of label, agree very closely with those obtained with 51Cr. Loss of label in the first hour after injection is only about 4% which is significantly less than that observed with a post-tinning procedure. Because of the much lower rate of toxicity of 99mTc compared with 51Cr, 99mTc-labelled red cells should be used for estimating red cell volume whenever practicable.