The propagation of hepatitis A virus (HAV), CF53 strain, released without any cytopathic effect into the PLC/PRF/5 cells supernatant, was studied in the course of six serial passages (6th to 11th). The decrease (from 5 to 1 week) of incubation time required to detect HAV, by RIA, in culture supernatant, the increase in Hepatitis A antigen (from 777 to 10,038 c.p.m./50 microliter) and infectivity titre (from 10(3.0) TCID 50/ml to 10(4.5) TCID 50/ml) were consistent with the adaptation of this virus to the cell line PLC/PRF/5.