Cell death induced by polychlorinated biphenyls (PCBs), environmental toxicant, was investigated in mouse splenocytes. The fragmentation of intact DNA, a parameter of apoptotic cell death, was evaluated qualitatively by agarose gel electrophoresis analysis and quantitatively by diphenylamine reaction method. PCBs induced apoptotic cell death of splenocytes in a dose- and time- dependent manner. The effect of serum on the apoptotic cell death induced by PCBs was also investigated. The DNA fragmentation induced by PCB treatment in serum-free medium was clearly inhibited by an addition of serum to the culture medium. The decrease of DNA fragmentation due to serum addition was accompanied with the increase of cell viability.