The response of musculocutaneous and random-pattern flaps to bacterial inoculation was studied in dogs by intradermal injection of bacteria and deposition of bacteria into stable wound spaces created by wound cylinders. No difference could be demonstrated in the susceptibility to bacterial challenge in the different portions of the musculocutaneous flap and in normal skin. A larger area of necrosis was observed in the random-pattern flaps, and the distal part was significantly more susceptible to necrosis. While the musculocutaneous flaps recovered rapidly from the bacterial inoculation, necrosis was observed in the random-pattern flaps. The bacterial count increased in the wound spaces surrounded by the random-pattern flaps, leading to full-thickness flap necrosis. The bacterial count decreased in the wound spaces surrounded by musculocutaneous flaps; there was evidence of healing around the wound cylinders. The musculocutaneous flap demonstrates a greater resistance to bacterial inoculation than the random-pattern flap on both its cutaneous and muscular surfaces.