Objective: The aim of this study was to evaluate and compare in vitro biocompatibility and poly(glycolic acid) (PGA) and poly(lactide-co-glycolide acid) (PLGA) on tumorigenic and non-tumorigenic mature spheroids. Material and Methods: This is an in vitro experimental study. Tumorigenic (C6 glioma, SH-SY5Y, MDAH2774, MCF-7) and non-tumorigenic cells [CRL11372, primary osteoblasts (MCPO)] as well as their mature spheroids were cultured alone as a control group as well as in combination with PGA and PLGA. Total cell numbers, bromodeoxyuridine labeling index (BrDU-LI), apoptosis, morphology, and ultrastructure were evaluated. Results: PGA and PLGA significantly decreased the number of SH-SY5Y and C6 glioma cells; MDAH 2774 cells also decreased, but not significantly (p> 0.05). Low BrDU-LI (p< 0.05) with a high level of apoptosis (p< 0.05) at C6 glioma and a high level of BrDU-LI (p< 0.05) with a low level of apoptosis at MDAH2774 (p< 0.01) were noted. These biopolymers mostly decreased the number of CRL-11372 cells (p< 0.05), but indicated an increased apoptosis (p< 0.01) and significant BrDU-LI (p< 0.05). Biopolymers induced chromatin condensation (typical apoptotic ultrastructure) and vacuolization primarily at SH-SY5Y spheroids but rarely at MDAH-2774 spheroids. This apoptotic ultrastucture was most often observed at MCPO spheroids. PLGA and PGA induced similar BrDU-LI decreases among tumorigenic spheroids (p< 0.05), although this decrease was greater at MCF-7 (p< 0.05) in the PGA group. PGA primarily decreased BrDU-LI at CRL 11372 (p< 0.05), although the decrease was almost identical to that at MCPO for the two biopolymers (p< 0.05). A significant attachment affinity was determined at MDAH -2774 and C6 glioma spheroids. Conclusion: This study demonstrated the biocompatibility of PGA and PLGA at mature spheroids of tumorigenic and non-tumorigenic cell lines, which changed according to the cell type.