A 69,XXX female liveborn triploid survived 45 days. The phenotype was consistent with the average clinical picture of liveborn triploids. Autopsy revealed slight atrophy of cerebral cortex and corpus callosum, and severe adrenal hypoplasia. Chromosome polymorphisms indicated that the origin of this triploid was dispermy. Replication studies of the X chromosome performed on lymphocytes and fibroblasts showed that the majority of cells had 2 late replicating X chromosomes. X chromosome inactivation in spontaneous abortuses and liveborn triploids is discussed. Nine enzymes [inorganic pyrophosphatase, hexokinase, esterase D, adenosinedeaminase, sphingomyelinase, .beta.-glucuronidase, arylsulfatase A, .alpha.-mannosidase and .beta.-galactosidase] encoded by autosomal genes were tested, 5 had normal, 3 increased and 1 reduced levels of activity. The reduced activity of .alpha.-galactosidase, an X-linked enzyme, is an agreement with cytogenetic findings and demonstrated a gene dosage effect.