In a model of immune complex glomerulonephritis in BALB/c mice, cells of monocyte/macrophage lineage (M phi), identifiable by electron microscopy, infiltrate the glomerulus. In spite of this, no unequivocal nonspecific esterase activity can be demonstrated histochemically in the glomeruli. On the other hand, many mononuclear cells with strong acid phosphatase activity are consistently present. This observation is in line with other studies that have demonstrated the heterogeneity of enzyme profiles in different M phi populations. Despite the wide acceptance of nonspecific esterases as markers for M phi, the present study indicates that exclusive reliance on a single marker in investigating the participation of M phi in a pathological process can lead to erroneous conclusions. Use of multiple markers and preferably multiple investigative modalities is recommended.