Abstract.Monocytes of seven out of eight patients with type 1 C1‐inhibitor (C1‐inh) deficiency (HAE) produced 40% as much C1‐inh as monocytes from normal donors (controls). In contrast, monocytes from three patients with type 2 and three patients with acquired C1‐inh deficiency produced similar amounts of Cl‐inh as controls. Recombinant γ‐interferon (γ ‐interferon 10 ng/ml) stimulated C1‐inh production of C1‐inh (eight‐10‐fold) by control and patients' monocytes. Monocytes from patients with type 1 HAE contained 40% the level of C1‐inh messenger ribonucleic acid (mRNA) found in control monocytes, γ ‐interferon increased the abundance of Cl‐inh mRNA by the same extent in both control and patients' monocytes. C1‐inh protein and mRNA were undetectable in the monocytes of one patient, unless stimulated by γ‐interferon. Under these conditions, his monocytes produced comparable amounts of C1‐inh (protein and mRNA) as γ‐interferon‐stimulated monocytes of the other type 1 HAE patients.The data suggest that in most type 2 HAE patients there is a lesion in the C1‐inh gene such