This study evaluates the safety, tolerability, pharmacokinetics, and maximum tolerated dose (within dosing range) of intravenous (IV) infusion of C2N-8E12 in 32 patients with progressive supranuclear palsy (PSP). Four sequential cohorts will receive increasing single doses of either C2N-8E12 or placebo. Out of every 4 patients enrolled 3 patients will receive drug and 1 will receive placebo. Study participants will be followed for a minimum of 2 months post-treatment to monitor for the safety, tolerability, pharmacokinetics, and immunogenicity of C2N-8E12.