This is a 73-day phase II, open label trial of the true human monoclonal antibody RA-18C3 in subjects with moderate to severe plaque psoriasis. Ten (10) subjects will receive 200 mg of RA-18C3 via subcutaneous injection. Subjects will receive injections at Days 0, 21, and 42 for a total of 3 injections. Study drug will be administered under close observation in a facility equipped to handle medical emergencies. Subjects will not be discharged from the facility until at least 1 hour following the end of the injection or 1 hour after their vital signs have stabilized. Safety will be assessed by pre- and post-treatment serial measurements of vital signs, clinical laboratory assessments, and the recording of adverse clinical events.