DISEASE CHARACTERISTICS: Newly diagnosed multiple myeloma (diagnosis within 12 months of study) and scheduled to undergo autologous peripheral blood stem cell transplantation Prior diagnosis of monoclonal gammopathy of undetermined significance (MGUS) or smoldering myeloma (SMM) allowed if the criteria for diagnosis of multiple myeloma was met within 12 months of study Serum or urinary M-protein confirmation of diagnosis (IgA, IgD, IgG, IgE, or light chain proteins) At least 10% plasma cells in bone marrow Must have received induction therapy without disease progression or relapse after initial response Prior induction therapy must have been completed no more than 6 months before stem cell collection and no more than 9 months before transplantation Must have undergone stem cell mobilization with cyclophosphamide IV and filgrastim (G-CSF) The following diagnoses are excluded: Non-secretory multiple myeloma IgM myeloma Solitary bone or extramedullary plasmacytoma Symptomatic MGUS or SMM Symptomatic indolent multiple myeloma

PATIENT CHARACTERISTICS: Age: 18 to 70 Performance status: ECOG 0-2 Life expectancy: Not specified Hematopoietic: See Disease Characteristics Hepatic:

Bilirubin no greater than 2 mg/dL SGPT no greater than 2 times upper limit of normal No clinical evidence of amyloidosis involving the liver Renal:

Creatinine no greater than 2.0 mg/dL Creatinine clearance at least 30 mL/min No clinical evidence of amyloidosis involving the kidney Cardiovascular:

LVEF at least 50% No evidence of amyloidosis on echocardiogram No uncontrolled arrhythmia No symptomatic cardiac disease Pulmonary: FEV1 at least 60% OR FVC at least 60% OR DLCO at least 60% No symptomatic pulmonary disease No clinical evidence of amyloidosis involving the lungs Other: HIV negative No cord compression No other concurrent illness that would preclude survival No clinical evidence of amyloidosis involving the autonomic nervous system or gastrointestinal tract No known allergy to vitamin C Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: See Disease Characteristics No prior thalidomide for myeloma No prior peripheral blood stem cell or bone marrow transplantation No concurrent thalidomide No concurrent interferon Chemotherapy: See Disease Characteristics No prior clarithromycin for myeloma No more than 2 courses of prior induction therapy containing an alkylating agent Endocrine therapy: No concurrent dexamethasone Radiotherapy: No prior radiotherapy to more than 20% of bone marrow No greater than 30 Gy to the spinal cord Surgery: Not specified Other: At least 28 days since prior bisphosphonates No prior new or experimental agents for myeloma No concurrent experimental therapies No concurrent bisphosphonates