Seroprevalence studies represent a very important tool to find out what fraction of population has already met with the new type of coronavirus (e.g . SARS-CoV-2). Without these data, it is almost impossible for the state authorities to manage the epidemic and adopt rational measures . This article brings the results of a medium-sized seroprevalence study which was carried out in the spring of 2020 in South Bohemia . In the Strakonice and Písek regions, the ELISA method was used to test the prevalence of IgA and IgG antibodies in 2011 subjects, volunteers from general public and selected professions working in areas with a higher exposure to the infection . The study showed that already in May 2020 , 2.9% of inhabitants of the Strakonice region and 1.9% of inhabitants of the Písek region had antibodies against the coronavirus . These numbers imply that for each PCR positive person, there were at least fifty others who had probably already undergone the infection . The article points out three types of problems that might occur in such a study . First, the study must be planned correctly, and possible outcomes must be pre-assessed . Second, an appropriate test must be selected with known parameters . This enables us to correctly estimate the share of false positive and false negative results . Third, the data must be evaluated in a reasonable way and correct inference must be performed . We offer a set of recommendations how to manage these issues and how to solve problems that inevitably arise in such a large-scale testing.