Early events in the infection of human B lymphocytes by Epstein-Barr virus (EBV) were examined by measuring calcium ion concentration from fluorescence with fura-2. Intracellular Ca ion concentration (Ca2+i) of B lymphocytes increased in response to EBV application. Three types of Ca2+i-increase were observed: (1) an early transient Ca2+i-increase; and (3) a slow Ca2+i-increase without the early transient Ca2+i-increase. The early transient increase was observed in the zero Ca2+ condition, but it was suppressed when cells were pretreated with ryanodine before exposure to the virus. The slow sustained Ca2+i increase was not observed in Ca(2+)-free extracellular conditions. These results suggest that the early transient Ca2+i increase is mediated by Ca2+ release from intracellular Ca storage sites, and the slow sustained Ca2+i increase is mediated by the Ca2+ influx through the plasma membrane. Virus receptors on the surface of B lymphocytes were stained with a fluorescence marker, rhodamine, and the capping process after EBV application was observed under a confocal microscope. The capping process and the localization of virus receptors were observed after EBV application. The time course of the capping process seems similar to that of the slow, sustained Ca2+i increase.