Implantation testing represents the ultimate biocompatibility assessment - placing materials directly in tissue reveals local responses that extraction testing cannot predict, demonstrating actual tissue integration or adverse reactions. USP Class VI intramuscular implantation testing per USP <88> evaluates tissue compatibility through direct implantation in rabbit muscle with histopathological assessment providing definitive biocompatibility evidence through microscopic examination. The protocol requires specific sample configurations surgically implanted with evaluation after 7, 30, or 90 days depending on intended clinical duration, capturing both acute inflammatory response and chronic tissue changes. Microscopic examination by qualified pathologists scores tissue responses using standardized criteria, encompassing acute inflammation indicating immediate tissue reaction, chronic inflammation suggesting ongoing adverse response, fibrosis representing encapsulation severity, and other tissue changes including necrosis or calcification. The pharmaceutical-grade requirements make this testing essential for drug-device combinations where material biocompatibility affects therapeutic efficacy, orthopedic implants requiring osseointegration rather than excessive fibrous encapsulation, and cardiovascular devices where fibrosis causes device failure or thrombogenic response. For biodegradable implants, serial implantation demonstrates degradation kinetics and validates that breakdown products don't cause unacceptable tissue reactions throughout absorption process. The direct tissue contact provides most clinically relevant assessment, revealing local toxicity, inflammatory potential, and tissue integration that in vitro testing cannot predict. Materials passing implantation testing demonstrate suitability for long-term tissue contact supporting regulatory submissions with definitive biocompatibility evidence, while failures indicate material reformulation necessity or processing modifications reducing tissue reactivity.