Viral respiratory tract infection (RTI), also known as common cold, is humanitys highest incidence illness and is caused by more than 200 different viruses. At Marinomed it was shown that iota-carrageenan, a sulfated polysaccharide, has an antiviral effectiveness against human rhinoviruses and human influenza viruses at a low concentration range of 0.01 – 100 µg/ml in vitro; it was also effective in clinical trials. Therefore, it is successfully used in two antiviral nasal sprays at concentrations of 1200µg/ml and 1600µg/ml, respectively. In this work, the antiviral activity of iota- and kappa-carrageenan against two important respiratory viruses, human coronavirus OC 43 (hCoV OC43) and human parainfluenza virus 3 (hPIV3) was investigated using two independent methods. A cellular infectivity reduction assay (IRA) was established successfully. This assay, together with a hemagglutination inhibition (HAI) assay, showed that both carrageenans inhibit the replication of hCoV OC43 and hPIV3 as well as their attachment to erythrocytes in a dose dependent manner at a similar range already reported for other viruses, with iota-carrageenan being more effective. Additionally, the effect of three zinc salts on the antiviral activity of iota-carrageenan was studied using the HAI assay. Here, no influence of zinc was observed.