The mouse H19 gene is expressed exclusively from the maternal allele. The imprinted expression of the endogenous gene can be recapitulated in mice by using a 14-kb transgene encompassing 4 kb of 5'-flanking sequence, 8 kb of 3'-flanking sequence, which includes the two endoderm-specific enhancers, and an internally deleted structural gene. We have generated multiple transgenic lines with this 14-kb transgene and found that high-copy-number transgenes most closely follow the imprinted expression of the endogenous gene. To determine which sequences are important for imprinted expression, deletions were introduced into the transgene. Deletion of the 5' region, where a differentially methylated sequence proposed to be important in determining parental-specific expression is located, resulted in transgenes that were expressed and hypomethylated, regardless of parental origin. A 6-kb transgene, which contains most of the differentially methylated sequence but lacks the 8-kb 3' region, was not expressed and also not methylated. These results indicate that expression of either the H19 transgene or a 3' DNA sequence is key to establishing the differential methylation pattern observed at the endogenous locus. Finally, methylation analysis of transgenic sperm DNA from the lines that are not imprinted reveals that the transgenes are not capable of establishing and maintaining the paternal methylation pattern observed for imprinted transgenes and the endogenous paternal allele. Thus, the imprinting of the H19 gene requires a complex set of elements including the region of differential methylation and the 3'-flanking sequence.