Mouse and human erythrocytes are inherently different with respect to slow dialysis encapsulation used in preparing carrier erythrocytes. Fluorescein isothiocyanate (FITC)-dextran was added to five different stages of the encapsulation process to discern when endocytic vesiculation occurred. Mouse erythrocytes were much more unstable than human cells, with as many as 50% of mouse cells showing vesicles, as determined by flow cytometry. Mouse erythrocytes showed the ability to form vesicles at each stage beyond the washed-packed-cell stage. Up to 20% of the human cells formed vesicles at stages beyond the annealed-cell stage. Although vesiculation can occur at any stage of the encapsulation process, with the exception of washed-packed cells, the actual amount of FITC-dextran incorporated in the cells is extremely low when compared with dialysis encapsulation, namely 50 ng/10(7) cells as against 2,000 ng/10(7) cells. Thus preparation of carrier erythrocytes by slow hypo-osmotic dialysis induces certain instabilities that lead to a substantial percentage of cells with endocytic vesicles, while the actual amount endocytosed is low. The differences in vesiculation observed between human and mouse erythrocytes is apparently related to the intrinsic properties of the cells and is consistent with the fact that mouse erythrocytes are more fragile when undergoing slow dialysis than are human erythrocytes.