A series of three ruthenium complexes, i.e. trans-dichlorote-trakisdimethyl-sulfoxide ruthenium(ll) (trans-Ru), imidazolium trans-imidazoletetra-chlororuthenate (ICR) and sodium trans-tetramethylensulfoxideisoquinoline-tetrachlororuthenate (TEQU), were studied in vitro in comparison to NAMI-A, a potent ruthenium-based antimetastasis agent. In vitro challenge of TS/A adenocarcinoma or KB oral carcinoma tumor cells with 10(-4) M concentration for 1 h evidenced the lack of cytotoxicity of NAMI-A, ICR and trans-Ru, the accumulation of cells in the G2/M pre-mitotic cell phase by NAMI-A and the attachment of tumor cells to the plastic substrate was significantly greater for NAMI-A than for ICR. These data stress that in vitro cytotoxicity is not necessary for in vivo activity of ruthenium antitumor complexes: NAMIA, ICR and trans-Ru, are in fact known to be active against murine tumors in the mouse system. Rather, TEQU, the compound free of in vivo activity, was the only one to reduce cell growth of in vitro cultured cells. In conclusion, the data on the effects of NAMI-A on in vitro cultured cells show that the increase of cell adhesion properties and the transient cell cycle arrest in the G2/M phase are much more relevant than the effects on cell properties relevant to cell growth (i.e. on CD44, CD54 or CD71 antigens) for determining in vivo antimetastasis activity.