Summary. -Human T lymphotropic virus 1 (HTLV-1) is a pathogenic retrovirus that spreads predominantly via cell-to-cell contact. Two models of cell-to-cell virus transmission are proposed: virological synapse (VS) and viral biofilms (VB). Both infectious structures can be involved in transmission and synergistically enhance HTLV-1 spread between cells. Although transmission of virus via VB has been reported, the molecular composition of VB remains poorly understood. In this study we generated new anti-VB monoclonal antibodies (MAbs) and screened them along with a panel of anti-human cluster of differentiation (CD) MAbs to select antigens associated with VB. Among four MAbs generated against VB, two MAbs were identified as anti-CD25 (IL-2RA). We found that antigens CD4, CD150, CD25, CD70, and CD80 were enriched in VB. We also determined that expression of viral protein Tax, a central molecule in HTLV-1 transmission, upregulates intercellular adhesion molecule 1 (ICAM-1), CD95, CD25, CD70, and CD80. Whether these antigens are essential for VB formation and HTLV-1 infection remains unknown and will be determined in further experiments.Keywords: monoclonal antibody; CD antigen; HTLV-1; biofilms * Corresponding author. E-mail: dvmazurov@yandex.ru; phone: +74996128854. Abbreviations: CD = cluster of differentiation; HIV-1 = human immunodeficiency virus 1; HTLV-1 = human T cell lymphotropic virus 1; ICAM-1 = intercellular adhesion molecule 1; IL-2-RA = interleukin 2 receptor subunit A; IP = immunoprecipitation; MAb(s) = monoclonal antibody(ies); MS = mass-spectrometry; PE = phycoerythrin; TM4SF = transmembrane 4 superfamily proteins; VB = viral biofilms; VS = virological synapse; WB = Western blot; Wt = wild type