We have investigated the compartmentalization of aminopeptidase-N-like activity in various blood fractions obtained from patients with acute lymphoid (ALL) or myeloid (AMD leukemia. The primary difference appears not to be the absolute level of overall activity, but rather the relative proportions of the different forms of activity detected. Thus, despite similar levels of total aminopeptidase-N-like activity detected in cells from different leukemic groups, true aminopeptidase-N/CD13 activity was only detected in cells derived from AML patients. Even in these patients, however, most of the detected aminopeptidase-N-like activity (>80 ) could not be attributed to aminopeptidase-N/CD13. In marked contrast, plasma from leukemic patients also contained substantial total aminopeptidase-N-like activity, of which (irrespective of leukemic group) most could be attributed to aminopeptidase-N/CD13. Whilst slightly higher levels of total activity were obtained in plasma from AML patients compared to ALL patients, there was no difference in the relative proportion attributable to aminopeptidase-N/CD13 (approximately 80 of total aminopeptidase-N-like activity). Evaluation of total aminopeptidase-N-like activity present in whole blood gave differential patterns, and whilst only a proportion (20-40 of total aminopeptidase-N-like activity) could be attributed to true aminopeptidase-N/CD13, blood from patients with CD13+ AML showed the greatest activity so attributable. In total, our results outline the complexities of peptidase activities present within blood of leukemic individuals, and may, in part, explain the variability of previous studies attempting to associate prognostic features with phenotypic expression of CD13.