Biocompatibility, sensing, and self-actuation are very important features for a therapeutic biomedical microrobot. As a new concept for tumor theragnosis, this paper proposes a monocyte-based microrobots, which are combining the phagocytosis and engulfment activities containing human acute monocytic leukemia cell line (THP-1) with various sized polystyrene microbeads are engulfed instead of a therapeutic drug. For the validation of the blood vessel barrier-penetrating activity of the monocyte-based microrobot, we fabricate a new cell migration assay with monolayer-cultured endothelial cell (HUVEC), similar with the blood vessels. We perform the penetrating chemotactic motility of the monocyte-based microrobot using various types of the chemo-attractants, such as monocyte chemotactic protein (MCP)-1, human breast cancer cell lines (MCF7)-cell lysates, and -contained alginate spheroids. The monocyte-based microrobot show chemotactic transmigrating motilities similar with what an actual monocyte does. This new paradigm of a monocyte-based microrobot having various useful properties such as biocompatibility, sensing, and self-actuation can become the basis of a biomedical microrobot using monocytes for diagnosis and therapy of various diseases. (C) 2014 Wiley Periodicals, Inc.