The present invention encompasses IL-12p40 binding proteins, particularly antibodies that bind human interleukin-12 (hIL-12) and/or human IL-23 (hIL-23). Specifically, the invention relates to antibodies that are chimeric, CDR grafted and humanized antibodies. Preferred antibodies have high affinity for hIL-12 and/or hIL-23 and neutralize h IL-12 and/or hIL-23 activity in vitro and in vivo. An antibody of the invention can be a full-length antibody or an antigen-binding portion thereof. Method of making and method of using the antibodies of the invention are also provided. The antibodies, or antibody portions, of the invention are useful for detecting hIL-12 and/or hIL-23 and for inhibiting hIL-12 and/or hIL-23 activity, e.g., in a human subject suffering from a disorder in which hIL-12 and/or hIL-23 activity is detrimental.