 RATIONALE: Biological therapies, such as anti-thymocyte globulin, may stimulate the immune system in different ways and stop cancer cells from growing. Drugs used in chemotherapy, such as melphalan, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Anti-thymocyte globulin may also make cancer cells more sensitive to melphalan. Giving anti-thymocyte globulin together with melphalan may kill more cancer cells. 