A serological study of 433 university students in various health care professions revealed levels of susceptibility of 2-7% for measles, 0-4% for mumps, 6-17% for poliomyelitis and a particularly disquieting 13-23% for rubella. Representing a developed population with a supposedly high level of immunisation cover these levels of susceptibility indicate students' vulnerability to outbreaks caused by viruses coming from reservoirs in under-immunised developing populations in the country. Nevertheless, no evidence of occupational exposure between the first- and final-year students was revealed. Immunisation efforts thus need to be urgently targeted not only to developing but also to developed populations in South Africa.