Invasive meningococcal diseases have become in the Czech Republic since 1993 a serious epidemio- logical and clinical problem due to a clonus which was not present previously: Neisseria meningiti- dis C:2a:P1.2,P1.5, ET-15/37. In 1996 a trial was conducted focused on the problem how this altered epidemiological and clinical situation is reflected in carriership of Neisseria meningitidis and Neisseria lactamica in the healthy population. Two age groups were followed up which were most severely affected by the new clonus of the meningococcus: 15–19 years (410 subjects) and 1–4 years (116 subjects). The trial was implemented in Olomouc where in 1993 the new epidemiological situation of the incidence of the invasive meningococcal disease was so serious that targeted vaccination was introduced. Of 116 children in the age group from 1–4 years in none Neisseria meningitidis was detected, in 9 Neisseria lactamica was found (7.7%). On repeated examination of children with a positive cultivation of Neisseria lactamica after two weeks in none Neisseria meningitidis nor Neisseria lactamica were found. Of 410 subjects in the age group from 15–19 years in none Neisseria lactamica was detected and in 35 Neisseria meningitidis (8.5%). Examinations were repeated after two weeks in 33 carriers: in 31 Neisseria meningitidis was again cultivated. Analysis of factors influencing carriership revealed in Neisseria lactamica two factors in young children which significantly promote this carriership: cold and close contact/kissing. A risk factor at the limit of significance are frequent respiratory diseases. In the carriership of Neisseria meningitidis in 15–19 year-old subjects six factors were revealed which promote carriership. A significant risk factor is close contact/kissing, the existence of partnership, participation in activities of the „disco“ type, living in a town, flats in the centre of the town. Effort is a risk factor at the limit of significance.

        Key words: Neisseria meningitidis – Neisseria lactamica – carriership – risk factors.