There are different recommendations for the handling of blood samples for analyses of the kallikrein-kinin or complement system, respectively. C1 inhibitor (C1-INH) takes a crucial part in both systems. In order to establish recommendations for blood specimen collection and transport for making the diagnosis of hereditary angioedema (HAE), the effect of time, temperature and different additives on C1-INH function and antigen was determined. We used blood samples from normals and patients suffering from HAE type I. Plasma containing EDTA, heparin, sodium citrate or polybrene-EDTA, and serum were assayed after incubations at 4 degrees C or 37 degrees C for 6 or 24 h. In addition, pooled serum was incubated for up to 5 days at room temperature. A modest decrease in C1-INH function was observed as an effect of storage-time in samples from normals (p = 0.039) and a substantial decrease was seen for the HAE patients (p = 0.0002). No significant effect of temperature (4 degrees C or 37 degrees C) was found. Clotting did not reduce C1-INH activity. Plasma containing heparin or polybrene interfered with the functional assay, yielding falsely high and low values, respectively. C1-INH functional assay performed within 24 h in serum, EDTA-treated or citrated plasma discriminated well between HAE patients and normals. This was also the case for serum kept at room temperature for up to 5 days, although a modest fall in C1-INH function was seen in the incubation period. For practical purposes we recommend serum as the sample of choice, preferably received within 48 h.