A possibility of in vitro labeling of erythrocytes with 99mTc using a kit of reagents (on the basis of DTPA, pyrophosphate, oxabifor, phytine) was studied. Optimum amounts of bivalent Sn [correction of Pb] were established for the reagents under study, and optimum kinetic parameters of labeling were defined. The yield of labeled erythrocytes under optimum conditions was over 95%.