We studied a baby born with physical features suggestive of the aminopterin syndrome, but without exposure of the mother to aminopterin during pregnancy. G-banded chromosomes from peripheral blood lymphocytes had a normal 46,XX pattern. However, in 50 skin fibroblasts there was a normal female karyotype in 5 cells and 45 cells showed an apparently balanced reciprocal translocation involving the long arm of chromosome 5 (band q35) and the long arm of chromosome 10 (band q22). The relation of this mosaicism to the abnormal phenotype is unclear