The presence of a null gene for alpha1-antitrypsin was detected in a family study by the inheritance of intermediate antitrypsin deficiency in association with a normal (PiM) phenotypic pattern. The proband, a 42-year-old man (M-phenotype), was a cigarette smoker and had physiologic evidence of pulmonary emphysema. Three female members of the family were receiving estrogenic medication but had deficient values for serum trypsin inhibitory capacity nevertheless, indicating an unresponsive gene. The mean serum trypsin inhibitory capacity for those with an M-phenotype was significantly lower than that found with an MZ phenotype, presumably due to the total noncontribution to serum antitrypsin activity by the null gene. A quantitative measurement of antitrypsin activity or concentration is necessary in an antitrypsin screening program, since phenotyping procedures alone cannot reveal the null gene.