The present invention relates to the ability of SAP to suppress fibrocytes. It also relates to the ability of IL-12, laminin-1, cross-linked IgG and IgG aggregates to suppress fibrocytes. Methods and compositions for suppressing fibrocytes using these proteins are provided. These methods are useful in a variety of applications including treatment and prevention of fibrosing diseases such as scleroderma, pulmonary fibrosis and asthma. Finally, the invention includes assays for detecting the ability of various agents to modulate differentiation into fibrocytes. Such assays may also be used to diagnose scleroderma, pulmonary fibrosis, or other fibrosing diseases.