The lipophilic complex, 99Tcm-hexamethylpropyleneamine oxime (HMPAO) is an efficient leucocyte label, and labels granulocytes with more stability than mononuclear leucocytes. The recovery of 99Tcm-HMPAO granulocytes, expressed as the percentage of injected granulocyte-associated activity circulating as granulocyte-associated activity 40-45 min after injection, was 37% (S.E. 3%), similar to the recovery of 111In-labelled granulocytes isolated and labelled in plasma using tropolone. The T1/2 of 99Tcm-HMPAO labelled granulocytes in blood was 4.4 h (S.E. 0.4 h), less than that of 111In-labelled granulocytes, although when a correction was made for 99Tcm elution, it was 6.4 h. The initial biodistribution of 99Tcm-labelled leucocytes was similar to 111In-labelled granulocytes, with a rapid initial lung transit, prominent splenic activity, bone marrow activity and minimal hepatic activity, although, unlike 111In, 99Tcm activity was also seen in urine, occasionally in the gallbladder, and, from about 4 h, consistently in the colon. Bone marrow activity was particularly prominent with 99Tcm. About 6% of 99Tcm was excreted in the faeces up to 48 h after injection, and about 17% in urine up to 24 h. The time-activity curves of reticuloendothelial activity up to 24 h were broadly similar for the two labelled cell preparations, and the differences that were observed can be explained on the basis of a higher rate of 99Tcm elution. Clinical information given by the two agents was similar in 27 of 30 patients who received both. Of the three who gave different information, one received 111In-labelled granulocytes which were considered to be functionally suboptimal and two, with inflammatory bowel disease, showed different distributions of abnormal bowel activity. We conclude that with respect to granulocyte kinetics and clinical data, 99Tcm-HMPAO labelled leucocytes are comparable with 111In-tropolonate labelled granulocytes.