We have cloned the functional gene coding for the L ferritin subunit by successive rounds of screening of a mouse genomic library using different oligonucleotides so as to avoid cloning the multiple pseudogenes of this rather complex multigene family. The L gene consists in 4 exons interrupted by 3 introns and spanning 1.8 kb. Quantitative measurements of H and L ferritin mRNA in various mouse tissues using a ribonuclease protection assay reveals important variations in the L/H ratio, the liver displaying the highest amount of L mRNA. Functional analysis of 1 kb of upstream sequence by transient transfections into the hepatoma cell line HepG2 shows that the mouse L gene transcription relies upon a minimal 130 bp promoter region containing 1 TATA box and 2 CCAAT motifs. Elements with an enhancing activity specific of hepatic tissue are likely to be located outside of this 1 kb fragment.