Lipoperoxide production by human monocytes was assessed during phagocytosis of opsonized zymosan particles. In this study, determination of lipoperoxides was performed by a fluorometric method coupled with solubilization of monocytes with sodium dodecylsulfate. Lipoperoxide production of monocytes was significantly lower in diabetic subjects than in normal subjects. Lipoperoxides are reported to have bactericidal activity and these results may indicate impaired bactericidal function of monocytes in diabetic patients.