A number of recent developments in research on blood cell labeling techniques are presented. The discussion relates to three specific areas: 1) A new in vitro method for red blood cell labeling with 99mTc, 2) a method for labeling leukocytes and platelets with 99mTc, and 3) the use of monoclonal antibody technique for platelet labeling. The advantages and the pitfalls are examined in the light of available mechanistic information. Problems that remain to be resolved are reviewed. An assessment is made of the progress as well as prospects in blood cell labeling methodology including that using the monoclonal antibody approach.