Ca(2+) was found to be essential for initiating Halobacterium salinarum CCM 2090 cell aggregation. The floc formed from such aggregation could easily be dissociated without cellular lysis by sodium citrate. Cr(2+), Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) could replace Ca(2+). However, Mg(2+), Sr(2+), Mo(2+), Cd(2+), Sn(2+), Hg(2+), and Pb(2+) induced no flocculation of cells of this halophilic archaeon. Mg(2+) acted antagonistically against Ca(2+)-induced aggregation. Such aggregation might be directly caused by the interaction of Ca(2+) and aggregation factors from 55 degrees C-treated cell extract.