The effect of treating a commercial skeletal imaging kit containing hydroxyethylidene diphosphonate (HEDP) with dimethyl sulfoxide (DMSO) prior to labeling with sodium pertechnetate, 99mTc[TcO4-], was investigated. Statistically, significant differences (P less than 0.05) in soft tissue retention (blood and muscle) were seen in rats after injection with the DMSO-treated HEDP compared to the nontreated HEDP. Based on i.r. and HPLC data, it appears that DMSO acts as an extractant for certain Sn2+-HEDP complexes which contribute to greater soft-tissue retention.