The alpha-1-antitrypsin (α1AT) Null alleles are those for which no α1AT can be detected in the serum attributable to the gene. The intracellular consequences of the various substitution, deletion, and insertion mutations causing the Null state can be categorized into two groups: those associated with detectable α1AT mRNA transcripts and those with no detectable α1AT mRNA transcripts. To classify the intracellular mechanism associated with the Null(granite falls) allele (Tyr160 TAC, C deletion, 5' frameshift → Stop160 TAG), a Null(granite falls) homozygote was evaluated. Genotypic diagnosis of the Null(granite falls) homozygous state was determined using the polymerase chain reaction and Null(granite falls) specific primers. Total cellular RNA extracted from alveolar macrophages of the index case was compared to that from a normal M1 homozygote for the presence of α1AT mRNA transcripts using Northern blot analysis and hybridization to either a 32P-labeled full length α1AT cDNA probe or (as a control) a 32P-labeled γ-actin cDNA probe. Although the macrophages of both the Null(granite falls) homozygote and the normal showed γ-actin mRNA transcripts in comparable amounts, Null(granite falls) macrophages contained no detectable α1AT mRNA transcripts whereas the normal had the expected 1.8 kb α1AT mRNA transcripts whereas the normal had the expected 1.8 kb α1AT mRNA transcripts. Thus, the Null(granite falls) allele can be classified along with Null(bellingham) as a Null allele associated with no detectable α1AT mRNA. These observations highlight the marked heterogeneity in the molecular processes causing the Null state, despite an identical phenotype at the clinical level.