The kinetics of human autologous granulocytes, separated and labelled with 111In without isolation from plasma, have been studied in subjects with and without sepsis with the aim of identifying the fate and sites of destruction of granulocytes in man. In subjects without inflammatory disease, 111In granulocyte recovery in faeces, urine and saliva over 4 d was less than 1% of the dose, so that the activity visualized by the gamma camera represented almost 100% of the dose. On images taken at 24 and 48 h, this activity was distributed between spleen, bone marrow and liver, with foci of additional abnormal activity in subjects with inflammatory disease. Splenic activity fell between 40 min and 24 h, consistent with the presence of a splenic granulocyte pool, but remained constant after 24 h. Since granulocyte clearance from the blood was predominantly completed by 24 h, the residual splenic activity at that time reflected splenic granulocyte destruction. In patients with sepsis, the fall in splenic activity was greater than in those without, implying diversion of granulocytes from splenic destruction to tissue utilization when inflammation is present. Bone marrow activity increased between 40 min and 24 h and then remained stable. Granulocytes that were extensively manipulated in saline prior to labelling failed to localize in marrow, suggesting that visualization of the latter reflected destruction of intact, normal granulocytes. Although the changes in splenic and marrow activities terminated at 24 h, at which time granulocyte clearance from blood was at least 80% completed, plasma 111In remained essentially unchanged between 40 min and 48 h at less than 5% of the dose, discounting it as the source of splenic and marrow activities.