MM cells produce ammonium in excess in the presence of Gln. RPMI 8226, OPM2, JJN3, KMS-12-BM, XG1, and 697 cells were seeded at 5 × 10⁵ cells/mL in RPMI-1640 with 10% FBS in the presence (w) or absence (w/o) of Gln (4 mM) and cultured for 16 hours. CMs were collected and immediately analyzed. The CMs of primary BM CD138⁺ and CD138⁻ fractions of 10 MM patients were also obtained, following the same procedure. Then NH₄⁺ levels were evaluated. (A) Bar graph represents the mean NH₄⁺ plus standard deviation (SD) secreted by cell lines in 2 independent experiments (2-tailed unpaired t test, *P < .05; **P < .01). (B) Plots represent the single values of NH₄⁺ secreted by HMCLs, BM CD138⁺ MM cells, and BM CD138⁻ cells. (C) Plots represent the single values of BM plasma NH₄⁺ of patients affected by MGUS and SMM (n = 13) and by active MM (n = 17). (D) Gln and (E) glutamate (Glu) in BM plasma of patients with indolent monoclonal gammopathies (MGUS and SMM; n = 10) or active MM (n = 13), evaluated by high performance liquid chromatography. For B-E, lines represent median values for Mann-Whitney test.