We investigated, via multicolor flow cytometry, the in vivo effects of colony‐stimulating factors (CSFs) on cell size, frequencies, and expression of surface antigens on peripheral blood monocytes from melanoma patients treated concurrently with CSFs and tumor‐specific monoclonal antibody (mAb) R24. Recombinant human macrophage colony‐stimulating factor (rhM‐CSF) increased cell size, relative percentages of monocytes, percentages of CD14⁺, HLA‐DQ⁺, CD11b⁺, and CD16⁺ monocytes, and cell‐surface expressions of HLA‐DR and CD11b; rhM‐CSF also up‐regulated cell‐surface expression of CD14 on CD14^brightCD16⁻ monocytes. Recombinant human granulocyte‐macrophage colony‐stimulating factor (rhGM‐CSF) increased cell size, percentages of CD14⁺, HLA‐DQ⁺, and CD11b⁺ monocytes, and cell‐surface expressions of HLA‐DR, HLA‐DQ, CD11b, and CD58. Relative percentages of monocytes and CD16⁺ cells and cell‐surface expression of CD¹⁴ on CD14^brightCD16⁻ monocytes decreased. In addition, monocytes derived from patients treated with rhM‐CSF showed functional activity when assayed in vitro for antibody‐dependent cellular cytotoxicity (ADCC). During treatment and coincident with increased CD16 expression, monocytes derived from rhM‐CSF patients had enhanced levels of cytotoxicity towards melanoma target cells compared to healthy controls and to patients treated with rhGM‐CSF. © 1995 Wiley‐Liss, Inc.