The new sodium bis(1,2,4-triazol-1-yl)acetate ligand, Na[HC(CO(2))(tz)(2)], has been prepared in methanol solution by using 1,2,4-triazole, dibromoacetic acid, and NaOH. Treatment of the [Cu(CH(3)CN)(4)][PF(6)] acceptor with Na[HC(CO(2))(tz)(2)] or Na[HC(CO(2))[(pz(Me2))(2)] in the presence of the tris(hydroxymethyl)phosphine coligand in methanol/acetonitrile solutions produced unprecedented mononuclear copper(I) complexes of the [L(n)]Cu[P(CH(2)OH)(3)](2) (L(1), 2; L(2), 3) [(CH(3)CN)(2)Cu(P(CH(2)OH)(3))(2)]PF(6), 4. These compounds have been characterized by elemental analyses, FTIR, ESI-MS, and multinuclear (1H and 31P) NMR spectral data. The new copper(I) complexes were tested for their cytotoxic properties against a panel of several human tumor cell lines. The results reported here indicate that all the complexes showed in vitro antitumor activity similar or better than that of cisplatin, the most used metal-based antitumor drug. In particular, [HC(CO(2))(pz(Me2))(2)]Cu[P(CH(2)OH)(3)](2), 3 showed IC(50) values markedly lower than the reference compound against all tumor cell lines. Chemosensitivity tests performed on cisplatin sensitive and resistant cell lines have demonstrated that all these Cu(I) complexes were able to overcome cisplatin resistance, supporting the hypothesis of a different mechanism of action compared to that exhibited by the reference drug. Flow cytometric analysis on 2008 human ovarian carcinoma cells revealed that complex 3, chosen as the best candidate, induced a marked enlargement of both cell size and granularity, and a significant increase in the fraction of G2/M cells that, differently from cisplatin, was not accompanied by the appearance of a relevant sub-G1 fraction. Besides, no evidence of caspase-3 activation was detected in cells treated with complex 3. We hypothesize that the cytotoxic activity of the new copper(I) complex may be correlated to its ability to trigger paraptosis, a nonapoptotic mechanism of cell death.