Oncolytic viruses hold promise as "self-amplifying" cancer therapies wherein a virally killed cell can produce thousands of new viral "drugs" that can kill more cancer cells. Adenoviruses (Ads) are one family of oncolytic viruses. Most human studies have used human Ad serotype 5 (Ad5). Unfortunately, most patients are already immune to Ad5 increasing the likelihood that the agent will be neutralized if used as a cancer therapy. In this work, lower seroprevalence Ad6 was tested as a systemic therapy for prostate cancer. Ad5 and Ad6 were injected intravenously a single time in nude mice bearing human prostate tumors, and toxicity and efficacy were assessed. Ad6 was chemically shielded with polyethylene glycol (PEG) to test if this would further improve its pharmacology. Ad6 produced 30-fold lower liver damage and less toxicity than Ad5. Ad6 significantly repressed the growth of androgen-resistant human DU145 prostate tumors and androgen-sensitive LNCaP tumors after single intravenous injection. PEGylation did not change virus distribution, but blunted liver damage and cytokine production by Ad6. PEGylated Ad6 eradicated LNCaP tumors and maintained body mass, but lost potency against the more challenging DU145 tumors. These and other data suggest that low seroprevalent Ad6 has better efficacy and safety than the benchmark oncolytic virus Ad5 for systemic therapy of prostate cancer. These data also indicate that PEGylation may improve Ad6 safety, but that this shielding may reduce oncolytic efficacy after intravenous treatment.