To provide a method for monitoring the tumoricidal function of isolated, virtually pure human monocytes cultured in suspended state in a serum- free medium and converted into active killer monocytes.

This monitoring method comprises the following two successive steps: assessment of the tumoricidal activity of active killer monocytes based on the formula: A/T×100 (A is 111In count number emitted from a test well; T is the whole 111In count number taken in target cells), and assessment of the specific tumoricidal activity of the active killer monocytes based on the formula: (B-C)/(T-C)×100 (B is 111In count number emitted from a test well involving monocytes and target cells in the presence or absence of an activator; C is 111In count number emitted from a test well involving target cells in the presence or absence of the activator same as the case in B).