Ca2+ was found to be essential for initiating Halobacterium salinarum CCM 2090 cell aggregation. The floe formed from such aggregation could easily be dissociated without cellular lysis by sodium citrate. Cr2+, Mn2+, Fe3+, CO2+, Ni2+, CU2+, and Zn2+ could replace Ca2+. However, Mg2+, Sr2+, Mo2+, Cd2+, Sn2+, Hg2+, and Pb2+ induced no flocculation of cells of this halophilic archaeon. Mg2+ acted antagonistically against Ca2+-induced aggregation. Such aggregation might be directly caused by the interaction of Ca2+ and aggregation factors from 55 degrees C-treated cell extract.