Platelet trapping was explored during the course of bleomycin induced pulmonary fibrosis by the injection of indium-111 labelled platelets and by light and electron microscopy (EM) of the alveolar capillaries. An i.v. injection of bleomycin markedly increased the localization of labelled platelets in the lung (but not in other organs) for about 3 weeks. On day 7 after bleomycin injection, a significant increase in the number of platelets in contact with the alveolar endothelium was seen with EM. Platelet trapping was strongly correlated (P < 0.005) with collagen deposition when examined in mouse strains genetically susceptible (CBA, C57BL/10, BL10 A.2R), or resistant (Balb/c, BL10.D2, BL10.A), to bleomycin induced fibrosis. In addition, several treatments known to decrease bleomycin induced collagen deposition and synthesis, namely administration of antibodies against CD11a, CD11b, TNF-alpha and IL-1ra, also decreased platelet trapping. As evaluated by EM, anti CD11a mAb significantly decreased the number of platelets in contact with the alveolar endothelium. This study indicates that bleomycin induced pulmonary fibrosis is strongly correlated with platelet trapping and that platelets probably interact, via their CD11a, with the CD54 born by the alveolar endothelium.