Human infection by BK virus (BKV), a human papovavirus isolated from the urine of a transplanted patient, occurs early in life and is very widespread. The clinical features of the primary infection are unknown. After the primary infection, BKV remains latent in the host tissues. Immunosuppression of the host induces the activation of the latent virus; BKV has been isolated only from immunosuppressed patients so far. In our study the incidence of BKV infection in immunosuppressed children and in children with normal immunological defenses was investigated both by antibody and virus research, the cultural research of the virus being performed in the urine and other specimens, such as nasal-pharyngeal swabs and cerebrospinal fluids. Our results show that the incidence of the primary infection in immunosuppressed children is comparable to the incidence in children with normal immunological defenses. Two antigenically identical viruses, which showed only partial correlation with BKV, were isolated from the urine of two immunosuppressed children. These two viruses were considered as a same antigenic variant of the BKV prototype. Of all the subjects examined, only 9 immunosuppressed children had antibodies to the newly isolated viruses.