The amniotic fluid of a 20 years old Caucasoid woman was brought in for analysis because of a previous child with anencephaly. Q-banded karyotypes disclosed 47,XY, +mar. Clinical significance of this karyotype could not be determined. The couple elected therapeutic termination but were denied because the pregnancy was over 23 weeks gestation. Her blood culture revealed also 47,XX, +mar. Silver staining revealed the marker chromosome bisatellited. C-banded karyotypes revealed the marker chromosome with one C-band. This inherited marker chromosome, composed of satellites, stalks, and constitutive heterochromatin without other observable chromosomal material present, suggested that it may be genetically inactive. A normal baby boy was delivered and his 47,XY, +mar karyotype confirmed. In addition to the family study, silver staining and the C-banding procedures are indicated to determine the morphology of satellites, stalks, and constitutive heterochromatin, and the presence or absence of other chromosomal materials in the extrasmall chromosome in amniotic fluid cultures.