A girl aged 9 months with clinical symptoms of Down’s syndrome had 47 chromosomes in preparations from peripheral blood leucocytes. An unusual karyotype was found with abnormal distributions of chromosomes in the following groups: C:17, D:5, F:3 and G:6. The interpretation of the karyotype is based on the assumption that there is a simultaneous occurrence of trisomy in group G and a translocation between chromosomes of group D and F. The mechanism to which these deviations are attributable is discussed. A causal relationship between trisomy and translocation is suggeste