Lymphocytes isolated from human peripheral blood were cultured in α minimal essential medium (α -MEM) supplemented with 10% fetal bovine serum and phytohemagglutinin (PHA) at 37°C for 68-72 h. The lymphocyte cultures were treated with BrdU to synchronize the cell population. The DNA probe was biotinylated with dATP using the Gibco BRL BioNick labeling kit and procedure for FISH detection was performed. The results showed that the number of cells with specific signal were 1(3), 2(70), chromatids per cell.