Background: Bone marrow-derived cell populations possess progenitor cell capacities. Emerging evidence also suggests significant plasticity of differentiated mononuclear cell lineages. We therefore assessed the distribution of transplanted peripheral blood mononuclear cells (PBMCs) in granulation tissue formation, and evaluated their possible transdifferentiation into myofibroblasts. Methods: Silastic tubes were inserted into the peritoneal cavity of rats, followed by injection of PKH26-labelled PBMCs isolated from donor animals. At 3, 14 and 21 days, the distribution of PKH26<sup>+</sup> cells as well as their colocalization with myofibroblast/smooth muscle cell [α-smooth muscle (α-SM) actin] or macrophage markers (ED1/ED2) were determined. Results: Round-shaped PKH26<sup>+</sup> cells accumulated around the implants at 3 days, while myofibroblasts were rare. Later, peritoneal granulation tissue constituted an inner, multilayered capsule primarily comprising α-SM actin<sup>+</sup> cells that was surrounded by more loosely organized inflammatory connective tissue. PKH26-labelled, spindle-shaped cells were abundantly found in tissue capsules. As a key finding, granulation tissue at 14 and 21 days contained cells with both PKH26 and α-SM actin labelling. Accordingly, a subpopulation of cells staining positive for macrophage markers showed a spindle-shaped morphology and α-SM actin expression. Conclusions: Transplanted PBMCs contribute to granulation tissue, and acquire myofibroblast characteristics during de novo tissue formation. Mononuclear cells may transdifferentiate into myofibroblast-like cells within an inflammatory environment.