The effect of influenza strains A (H3N2) and B, isolated during the seasons of 1994 and 1995 in the Czech Republic, on MDCK cells was studied. Various concentrations of virus and conditions of nutrition were used during the cell culture. The virus replication and consequently fragmentation of genomic DNA together with cytotoxicity were investigated in the absence and presence of 10 per cent calf serum. Virus replication, regardless of type A or B, caused earlier DNA fragmentation in comparison to non-infected cells in tissue culture. The results showed that the influenza B strain had a greater cytotoxic effect on MDCK cells than influenza A. A higher infection dose of influenza A virus accelerated the onset of apoptosis; conversely, a higher infection dose of influenza B virus delayed the onset of apoptosis. The absence of serum enhanced the progress of influenza-induced apoptosis in conditions in vitro.