The technique for detecting both foreign and host specific DNA sequences inside nuclei and chromosomes of single cells of transgenic mice with the help of polymerase chain reaction (PCR) in situ is described. The mouse preimplantation and postimplantation embryonic and adult cells were studied. The methodology is described in detail with particular attention to the optimization of composition of reaction mixture, kind of fixation and preliminary denaturation of target DNA. The reaction takes only several hours and needs no sophisticated equipment.