We have previously reported the cytotoxicity and antitumor activity of bis(diphenylphosphino)ethane (DPPE) and a variety of its transition metal complexes. During studies of the chemistry of a gold complex of this group [(AuCl)2(DPPE)], it was observed that this complex readily underwent ring closure on reaction with DPPE to form the tetrahedral complex [Au(DPPE)2]+. Various counterion forms (e.g., Cl-) of this cation were isolated and were found to exhibit a remarkably high stability in solution. Evaluation of [Au(DPPE)2]Cl in mice bearing i.p. P388 leukemia demonstrated that the compound produced an average of 87% increase in life span at its maximally tolerated dose (2-3 mumol/kg/day for 5 days). Activity was also seen in i.p. M5076 reticulum cell sarcoma (60% increase in life span) and s.c. mammary adenocarcinoma 16/c. Modest activity was evident in i.p. B16 melanoma and L1210 leukemia. A subline of P388 leukemia resistant to cisplatin was not cross-resistant to [Au(DPPE)2]Cl. In addition, combination therapy of [Au(DPPE)2]Cl and cisplatin against i.p. P388 demonstrated an advantage over single-agent therapy. In vitro studies of [Au(DPPE)2]Cl showed that the compound: is cytotoxic to tumor cell lines; is only minimally inhibited in its cytotoxic activity by the presence of serum; produces DNA protein cross-links and DNA strand breaks in cells; and inhibits macromolecular synthesis with a preferential inhibitory effect on protein synthesis relative to DNA and RNA synthesis. 31P nuclear magnetic resonance spectroscopy indicated that the compound is stable in the presence of serum proteins, thiols, or disulfides and that it reacts with Cu(II) resulting in the formation of a Cu(I)DPPE complex. The results of these in vivo and in vitro experiments suggest that the contrasting pharmacological profile of [Au(DPPE)2]Cl with respect to other gold(I) phosphine complexes may be related to both the kinetic stability of the complex and its stability in the presence of thiols.