The present invention relates to humanized immunoglobulins, mouse monoclonal antibodies and chimeric antibodies that have binding specificity for human CD52. The present invention further relates to a humanized immunoglobulin light chain and a humanized immunoglobulin heavy chain. The invention also relates to isolated nucleic acids, recombinant vectors and host cells that comprise a sequence which encodes a humanized immunoglobulin or immunoglobulin light chain or heavy chain, and to a method of preparing a humanized immunoglobulin. The humanized immunoglobulins can be used in therapeutic applications to treat, for example, autoimmune disease, cancer, non-Hodgkin's lymphoma, multiple sclerosis and chronic lymphocytic leukemia.