The (10)B enriched form of Na(2)B(12)H(11)SH (BSH) is used in a binary cancer radiation treatment, known as boron neutron capture therapy (BNCT). The BSH anion can also form a dimer, [B(24)H(22)S(2)](4-) (BSSB), previously used in animal model studies. It is demonstrated here that the retention of BSSB in mouse M2R melanoma and rat C6 glioma cells is significantly longer than that of the monomer BSH. The washout of the BNCT agents from cultured cells was followed using (11)B nuclear magnetic resonance spectroscopy. One hour after switching to boron-free culture medium, the relative amount of BSSB retained in M2R cells was at least about six times higher than that of BSH. The initial washout rate (over the first approximately 4 h) of BSSB was 4x10(-3) min(-1) (t(1/2), approximately 3 h) for M2R cells, and of the same order of magnitude for glioma cells.