Abstract Peripheral-type benzodiazepine binding sites (PBRs) are ubiquitous in mammalian tissues. However, the physiological role of PBRs has not yet been clarified. In this study we characterized a saturable and high affinity binding site for [ 3H]Pk 11195 (isoquinoline carboxamide derivative) on human lymphocytes and different lymphoma cell lines. Binding parameters of the human T-lymphoma cell line CCRF-CEM came closest to values for lymphocyte binding. Thus, the CCRF-CEM cell line appears to be a suitable lymphocyte cell model for further study of PBRs. To evaluate the pharmacological specificity of binding to human lymphocytes and CCRF-CEM cells we investigated the potency of different ligands to displace [ 3H]Pk 11195 from its binding site. Pk 11195 was found to be the most potent inhibitor followed by 4′-chlorodiazepan (Ro5-4864) and diazepam (range of inhibition constants from 6.7 × 10 −9M to 3.6 × 10 −7M), whereas ligands specific for the central-type receptor like clonazepam and flumazenil had no displacing potency in the tested concentration range (10 −10−10 −4M). Since it was assumed that PBRs might be involved in the regulation of cell growth and differentiation, we studied the influence of PBR ligands on cell growth and survival using a quantitative colorimetric assay (MTT). Ligands which bind selectively to PBRs inhibited cell multiplication in vitro. However, half-effective concentrations (EC 50) were in the micromolar range and above therapeutic in uivo concentrations (range of (EC 50) values from 2.4 × 10 −5M to 1.5 × 10 −4 M). Clonazepam and flumazenil had no inhibiting potency in the tested concentration range (10 −10−10 −4M). Although the difference between values for displacing potency and ability to inhibit cell multiplication cannot be explained as yet, it is interesting that all PBR-ligands followed the same sequence in displacing [ 3H]Pk 11195 and inhibiting cell multiplication and that central type ligands were ineffective in both assays. This association suggest a mediating role of PBR binding in cell growth.