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1. Direct ligand binding techniques have been used to compare bradykinin receptors in squamous- or adeno-carcinoma and healthy lung membranes removed from patients during operations. 2. The binding of [3H]-bradykinin to healthy lung membrane is time-dependent and saturable with a KD value of 1.08 +/- 08 nM and a Bmax value of 46.1 +/- 3.1 fmol mg-1 protein (n = 10). In squamous-carcinoma tissue (n = 8) the same amount of receptors are present, Bmax = 52.2 +/- 3.3 fmol mg-1 protein (P = 0.22) but the KD value is significantly higher 2.57 +/- 0.40 nM (P = 0.004). Similar measurements were obtained for adeno-carcinoma tissue (n = 3), KD = 2.80 +/- 0.29 mM (P = 0.001) and Bmax = 49.8 +/- 2.1 fmol mg-1 protein (P = 0.56). 3. In both healthy and squamous-carcinoma preparations, bradykinin analogues displace [3H]-bradykinin binding with the following relative order of potency: Hoe 140 > bradykinin > kallidin > D-Arg0[Hyp3,D-Phe7]bradykinin >>> des-Arg9-bradykinin. Of the analogues used, bradykinin and D-Arg0[Hyp3,D-Phe7]bradykinin appear to be able to differentiate the bradykinin receptors present in both preparations. 4. It is concluded that bradykinin receptors present in healthy and carcinomatous human lung are of the B2 type.