Introduction: Multiple myeloma (MM) is a malignant neoplasm of hematopoietic tissue classified as malignant neoplasms of mature B lymphocytes, which is the second most common blood cancer. The disease is caused by clonal neoplastic plasma cell proliferation (PC), resulting in extensive formation of non-functional gamma globulins or their parts. Immunoglobulin is a molecule built of two identical heavy chains and two light chains. Light chains are classified in kappa and lambda groups. Both chains are represented in all classes and subclasses of immunoglobulin, but in the same molecule both light chains are always equal, either kappa or lambda, and never kappa and lambda together. Research objective: The aim of this study is to determine the presence of free monoclonal light chains in multiple myeloma patients’ serum and to monitor the free chain concentration before and after cytostatic therapy, biological therapy and transplantation. Materials and Methods: This study included 20 patients recently diagnosed with multiple myeloma. Samples of MM patients’ serum were analysed to determine the concentration of free light chains of the immunoglobulin, kappa (ĸ) and lambda (λ) type by nephelometric analysis. The ĸ/λ ratio of free light chains of immunoglobulin was also determined. All parameters were measured twice, before and after the therapy. Results: There is a significant drop in the concentration of free light chain of both kappa and lambda type after the applied therapy, the kappa/lambda ratio did not significantly decrease. Conclusion: Analysis of free light kappa/lambda chains of immunoglobulin in serum of patients diagnosed with MM during the diagnosis and after the therapy may serve as an activity marker of the primary disease.