Purpose: Osteoarthritis (OA) is a common disease that will affect almost half the population at some point in their life through pain and decreased functional capacity. Numerous approaches have been proposed, but none can be considered an ideal procedure for the treatment of joint degenerative processes. Among the new therapeutic options, Platelet Rich Plasma (PRP), an autologous blood derived product, is emerging as promising intra-articular treatment for cartilage degenerative lesions. However, results on the potentiality of this biological treatment are still controversial and confounding, due to the lack of well-designed studies and to the different preparation procedures. Aim of this study was to increase knowledge concerning the biological effect of PRP treatment on clinically relevant joint cells. For this purpose, we compared the effect of two different PRP preparations on the expression of a panel of biomolecules by OA cultured synoviocytes. Methods and Materials: Blood collected from 7 volunteers was used to obtain two PRP preparations: a one-spinning procedure, aimed at obtaining a pure platelet concentrate (P-PRP) without leukocytes but with a limited number of platelets, and a two-spinning procedure, aimed at obtaining a higher platelet concentration but with the presence of leukocytes (L-PRP). Synovial tissues were collected from OA patients (Kellgren-Lawrence grade II- III) undergoing knee replacement surgery. Gene expression analysis of IL-1b, IL-6, IL-8, TNF, MMP-13, TIMP-1, -3, -4, VEGF, TGFb1, IL-10, IL-4,-13, FGF-2, HGF, Hyaluronic acid synthases (HAS)-1, -2, -3 was performed at day 7 (in agreement with weekly clinical administration of PRP) by semi- quantitative RT-PCR. HA (Hyaluronic Acid) production was analyzed in culture supernatants by ELISA. Statistical analysis was carried out using the General Linear Model. Results a) White blood cells (WBC) were present only in L-PRP, platelets concentrations were significantly higher in L-PRP than P-PRP. b) HA production and HAS gene expression did not seem to be modulated by PRP c) IL-1b, IL-8 and FGF-2 were significantly induced by L-PRP, whereas HGF was down-modulated by L- PRP compared to P-PRP. d) IL-1b and IL-8 expression levels significantly correlated with WBC count and showed a dose-response effect to L-PRP e) HGF mRNA expression showed an inverse correlation with both WBC and platelet count. f) MMP-13 expression inversely correlated with WBC number, whereas TIMP-4 inversely correlated with platelet count. h) HAS-2 and HAS-3 had respectively direct and inverse correlation trends with WBC count. Conclusion: Our data in OA synoviocytes indicate that L-PRP is able to sustain long-term up-regulation of pro-inflammatory factors, such as IL-1b, IL-8 and FGF-2 compared to P-PRP, together with a down modulation of HGF and TIMP-4 expression. These last two factors that have been recognized as anti-catabolic mediators in cartilage. Further researches are needed to clarify the influence of different concentrations of WBC on the bioactivity of PRP and their potential beneficial effect. Since WBC-platelet interaction may promote the biosynthesis of other factors that facilitate the resolution of inflammation, such as lipoxins, and given their involvement in immune-response, the optimization of their concentrations in PRP products might lead to minimizing or avoiding the detrimental effects ascribed to leucocytes and exploit their beneficial properties.