Purpose: The leukocyte- and platelet-rich fibrin block (L-PRF block) is a composite graft that combines a xenograft that is acting as a scaffold with L-PRF membranes that serve as a bioactive nodule with osteoinductive capacity. This study evaluated the properties of the L-PRF block and its components in terms of release of growth factors, cellular content, and structure. Materials and Methods: The concentration of transforming growth factor-β1 (TGF-β1), vascular endothelial growth factor (VEGF), platelet-derived growth factor-AB (PDGF-AB), and bone morphogenetic protein-1 (BMP-1) released by an L-PRF membrane and an L-PRF block were examined with ELISA for five time intervals (0 to 4 hours, 4 hours to 1 day, 1 to 3 days, 3 to 7 days, 7 to 14 days). Those levels in L-PRF exudate and liquid fibrinogen were also evaluated. The cellular content of the liquid fibrinogen, L-PRF membrane, and exudate was calculated. The L-PRF block was also analyzed by means of a microcomputed tomography (micro-CT) scan and scanning electron microscopy (SEM). Results: TGF-β1 was the most released growth factor after 14 days, followed by PDGF-AB, VEGF, and BMP-1. All L-PRF blocks constantly released the four growth factors up to 14 days. L-PRF membrane and liquid fibrinogen presented high concentrations of leukocytes and platelets. The micro-CT and SEM images revealed the bone substitute particles surrounded by platelets and leukocytes, embedded in a dense fibrin network. Conclusion: The L-PRF block consists of deproteinized bovine bone mineral particles surrounded by platelets and leukocytes, embedded in a fibrin network that releases growth factors up to 14 days. Keywords: blood platelets, cell count, fibrinogen, growth factors, scanning electron microscopy, tissue engineering