In 45 splenectomized individuals without malignancies, residual splenic tissue was detected by 99mTc-scanning using reinjection of labelled heat-damaged autologous erythrocytes. The angiotensin-converting enzyme in serum (SACE) and the blood monocyte count were measured in order to demonstrate an eventual association between these parameters as well as the influence of residual splenic tissue. In 20 patients with detectable spleen tissue SACE was normal (26.2 ± 4.4 U/ml), but in 25 patients without residual spleen tissue SACE was slightly elevated (28.7 ± 6.4 U/ml; p < 0.02) as compared with healthy controls (24.4 ± 6.2 U/ml). In both groups of patients blood monocytes were markedly increased. However, no correlation could be demonstrated between SACE and the monocyte count in any group. In 2 patients SACE was above the upper limit of the normal range ( > 36.8 U/ml) as found in sarcoidosis but no signs of this disease could be demonstrated. It is concluded that splenectomy must be taken into account when the significance of raised SACE is evaluated. It may be suggested that the spleen exerts an effect on the metabolism of SACE or on the ACE-producing cells in the organism.