Six metacentric non-satellite chromosome markers and 4 satellite markers of unknown origin were discovered by routine cytogenetic analysis. These markers were then investigated by isotopic and nonisotopic (FISH) in situ hybridization. In order to determine the origin of small marker chromosomes, a special protocol involving sequential application of defined alphoid and 'classical' satellite DNA probes with chromosome specificity was used. In situ hybridization under low stringency conditions has been performed with DNA probes specific for 4 groups of chromosomes. After preliminary analysis and determination of the possible origin of these marker chromosomes, DNA probes with high chromosome specificity were used under high stringency conditions. Marker chromosomes were found to be derivatives of chromosomes 7, 9 (3 cases), 13, 14 or 22, 21 (2 cases), X and Y.