Monoallelic expression of several genes has been observed in mice in which transcripts from parental homologues are distinguishable, but this phenomenon has not been demonstrated in humans. One monoallelically expressed murine gene, H19, encodes an abundant fetal RNA. We have found restriction site polymorphisms in the human H19 gene, located on chromosome 11p15, and examined the representation of these polymorphisms in cDNAs from fetal organs. Expression of H19 is largely or exclusively from a single allele; a similar analysis of the WT1 gene, on 11p13, shows biallelic expression. In the context of previous studies of 11p15 allelic losses in human embryonal tumours, our findings support the possibility of single-step inactivation of monoallelically expressed growth-regulating genes in human oncogenesis.