The binding of Tc-99m to neonatal red blood cells (RBC) was examined. Labeling efficiency was .apprx. 90%, and unbound Tc-99m > 3% after 1 washing, in premature and full-term newborns and in children. The presence of high percentages of fetal Hb (Hb F) did not influence the labeling of RBC with Tc-99m. RBC of 11 newborns were hemolysed and the distribution of Tc-99m on RBC components was analyzed. Although Hb F percentage averaged (60.0 .+-. 8.1)%, only (11.9 .+-. 3.7)% of Tc-99m was bound by Hb F, whereas (45.0 .+-. 6.1)% was associated with Hb A. RBC membranes bound (13.7 .+-. 4.3)% and (29.3 .+-. 4.0)% were unbound in hemolysates. Tc-99m preferentially binds to .beta. chains. In vivo equilibration of Tc-99m RBC and of albumin labeled with Evans blue was investigated in 5 newborn infants. Tc-99m RBC were stable in each case during the 1st h after injection. Elution of Tc-99m from RBC was (3.4 .+-. 1.5)% per hour. Body-to-venous hematocrit ratio averaged 0.86 .+-. 0.03.