Serum levels of beta 2-microglobulin (beta 2-M) have been assayed in 50 patients with multiple myeloma (MM) (stage 1-8, stage II-25. stage III - II and remission - 6). Smoldering MM was registered in 11 cases, active - 22 and aggressive - 17. Said level was significantly higher in MM patients than in healthy donors (p < 0,01) and correlated with stage and course in most cases: blood concentration of beta 2-M at stage I and in remission was found to be significantly lower than at stage II and III (p < 0,05, p < 0,05) while at stage III - higher than at stage II (p < 0,01). Serum level of beta 2-M was significantly higher in smoldering course than in healthy donors (p < 0,05) and lower than in active (p < 0,05) and aggressive MM (p < 0.01). Said concentration in blood proved significantly higher in aggressive course than in active MM (p < 0,05). Since some cases of active and aggressive MM revealed a low level of serum beta 2-M, reliance on this index as a sole prognostic test seems doubtful.