Resealed erythrocyte ghosts have been proposed as in vivo carriers for enzyme replacement therapy of inherited metabolic diseases. In comparative studies of methods for reversible hypotonic haemolysis of erythrocytes, a five-fold increased entrapment of human serum albumin was obtained by use of a dialysis procedure instead of direct dilution. The percentage incorporation of protein was also affected by varying mixing procedure, haematocrit, lysis, and resealing times but not by varying buffer composition or added protein concentrations over a wide range. Higher protein entrapment was observed with time-expired blood compared to fresh blood and this may be ascribed to increased osmotic fragility of the erythrocyte membrane in stored cells. Haemolysed and resealed ghosts prepared by any method used were smaller than normal erythrocytes as measured with a Coulter Counter and with a fluorescence-activated cell sorter, and protein entrapment reduced the ghost size further.