A 63-year-old patient complained about pain in the lumbar area. He underwent a course of physiological treatment on account of radiculitis but this led to no improvement of his condition. R-graphy of spinal column and pelvic bones revealed osteoporosis and serious bone defects. Blood analysis revealed moderate normochromic anaemia, urine analysis revealed proteinuria. Whole blood protein made up 10,7 g/l. What disease should be suspected?