Intraperitoneal (IP) injection is a method for administering drugs, vaccines, or biological agents into the peritoneal cavity. The protocol involves restraining the subject (e.g., mouse or rat), selecting the lower abdominal quadrant, cleaning the injection site with an antiseptic, and inserting a fine-gauge needle at a shallow angle to avoid organ puncture. The substance is injected slowly, and the needle is carefully withdrawn. This method is commonly used in preclinical research for drug delivery, immune studies, and pharmacokinetics.