A monoclonal antibody against human CD4, which antibody has a dissociation constant (K D) of less than 1×10 -9 M, binds to an extracellular region of human CD4 and is: (i) a recombinant human chimeric antibody in which the heavy chain variable region of the antibody comprises the amino acid sequence of SEQ ID NO:12 but excluding the secretory signal sequence of amino acids 1 to 19 in SEQ ID NO:12, and the light chain variable region of the antibody comprises the amino acid sequence of SEQ ID NO:22 but excluding the secretory signal sequence of amino acids 1 to 20 in SEQ ID NO:22; or (ii) a recombinant humanized antibody in which: (a) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:96 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (b) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:98 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (c) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:77 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (d) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; or (e) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:102. (i) a recombinant human chimeric antibody in which the heavy chain variable region of the antibody comprises the amino acid sequence of SEQ ID NO:12 but excluding the secretory signal sequence of amino acids 1 to 19 in SEQ ID NO:12, and the light chain variable region of the antibody comprises the amino acid sequence of SEQ ID NO:22 but excluding the secretory signal sequence of amino acids 1 to 20 in SEQ ID NO:22; or (ii) a recombinant humanized antibody in which: (a) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:96 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (b) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:98 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (c) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:77 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (d) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; or (e) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:102. (a) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:96 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (b) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:98 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (c) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:77 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; (d) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:78; or (e) the VH of the antibody comprises the amino acid sequence of SEQ ID NO:100 and the VL of the antibody comprises the amino acid sequence of SEQ ID NO:102.