Conditions for preparation of and the stability of carrier-red blood cells (C-RBC) are described for sheep RBC. The average percentage entrapment of 14C-sucrose was greater than 35% with 65% recovery of the cells. Cells prepared by dialysis encapsulation are morphologically similar to sheep RBC, although smaller in size than normal sheep RBC. C-RBC have a 28-day half-life in vivo and are stable in vitro for 24 hr at 37 degrees C. C-RBC are more resistant to osmotic stress than are normal sheep erythrocytes.