When the human coronaviruses, OC43 and 229E, are grown in the absence of calf serum, they have similar thermal inactivation rates and similar UV inactivation rates. When they are grown in the presence of serum, the thermal inactivation rate of OC43, and the UV inactivation rate of both OC43 and 229E are reduced, and evidence suggests this is because the viruses are in aggregates. The UV inactivation rate of serum-free OC43 and 229E is very similar to that of influenza A2 suggesting that the genomes are of comparable size. The shutoff temperatures for OC43 and 229E are 35-36ยฐ and 36-37ยฐ, respectively.