Studies using 3H-TdR, DF32P or 51Cr have all contributed important information concerning neutrophil kinetics in normal and abnormal conditions. Recent evidence, however, suggests that DF32P underestimates, and 51Cr overestimates, the blood neutrophil T1/2 and that both isotopes overestimate the TBNP, compared with 3H-TdR-derived data. The differences are quantitative and not qualitative, and the principles of blood neutrophil kinetics defined by DF32P studies are still valid. 3H-TdR studies are impractical for general use, and clinical measurement of neutrophil kinetics will have to continue to rely on the use of either DF32P or 51Cr. Comparison of abnormal findings with normal values obtained using the same isotope is probably valid for either technique. Changes in neutrophil kinetics leading to a neutrophil leucocytosis in different situations are fairly predictable from published data. In neutropenias, however, kinetic studies might be needed to delineate the relative contributions of under-production, shortened survival and excessive margination in the individual patient.