An EGTA (ethylene glycol bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) complex of the incomplete cubane-type, trinuclear tungsten sulfur cluster, [W3S4], was prepared and studied as a possible candidate for a new generation of x-ray contrast agents for medical diagnostic imaging. The aqua ion, [W3S4(H2O)9]4+ (I) was prepared through an improvement of the literature method to facilitate scale-up. Reaction of I with EGTA in refluxing DMF produced a mixture of (W3S4)x(EGTA)y (X,Y≥1) cluster complexes. A 2:3 (W3S4:ligand) complex, [(W3S4)2(EGTA)3]4- (II), was isolated via size-exclusion chromatog. in 25-40% yield. Cluster complex II was fully characterized by x-ray crystallog. in addition to routine techniques (crystal data for Na4(II).29H2O: space group P1̅, R = 2.81%). It is soluble in H2O and stable in the pH range 2 to 10. Animal studies of II indicated a low acute toxicity and a pharmacokinetic profile similar to that of an existing heavy metal cluster complex with a known extracellular biodistribution.