This paper explores the complexity of human ferritin H and L genes. We show that essentially full-length cDNA clones for human ferritin H and L chains do not cross-hybridize under moderate stringency conditions and present the first comparisons of H and L mRNAs and genes from the same species. Northern blot analyses indicate that the H and L mRNAs each contain about 1100 nucleotides. Subprobe analyses of Southern blots show that both H and L genes exist as multiple gene families. Both the 5' and 3' sequences of the H genes are heterogeneous, whereas the 3' end of the L gene is relatively conserved.