The objective of this work was to evaluate the binding characteristics of a cyclic peptide, cycle (1, 12)-Pen1-Pro2-Arg3-Gly4-Gly5-Ser6-Val7-Leu8-Val9-Thr10-Gly11-Cys12-OH (clBR), to Molt-3 T cells. This clBR peptide is derived from sequence numbers 11-20 of intercellular adhesion molecule-1 (ICAM-1). Binding studies were performed using a fluorescence-labeled peptide (FITC-clBR) in which the fluorescence marker fluorescein 5-isothiocyanate (FITC) was conjugated to the N-terminal of the clBR peptide. The binding affinity of the FITC-clBR peptide to Molt-3 T cells was evaluated using a FACScan flow cytometer. The binding specificity of the FITC-clBR peptide was also confirmed by inhibition of binding using unlabeled peptide (clBR). The results show that FITC-clBR binds to two populations of T cells with different affinities; population 1 has high cell numbers (75%) but low affinity, and population 2 has high binding affinity but low cell numbers (25%). Binding to both populations was saturable and could be inhibited by the unlabeled peptide (clBR), suggesting a receptor-mediated binding process. In addition to binding, receptor-mediated internalization was also observed for population 2; this was confirmed by confocal microscopy and temperature-dependence studies at 37 degrees C and 4 degrees C. The binding and internalization of this peptide may be carried out by surface receptors on Molt-3 T cells such as LFA-1. In the future, the binding and internalization of clBR peptide can be utilized as a method of targeted drug delivery to leukocytes for the treatment of leukocyte-related diseases.