The authors used the method of oblique illumination for detection of the state of the population of meningococcus cultures. In studying 48 strains there were revealed three types of fluorescence of the colonies: bright orange with a transition into greenish-light blue along the lower margin (the I type), bright light blue with a narrow orange or green upper margin (the II type) and greyish-light blue colonies (the III type). The type of fluorescence was not associated with the sero-group specificity. Populations of meningococcus cultures, depending on conditions and duration of growth on the nutrient medium could consist both of the colonies with the same type of fluorescence, and also represent a combination of colonies with a different type of fluorescence. The colonies of the I and II types of fluorescence had marked group specificity, but mutation occurred during passage and the III type of fluorescence formed with a loss of serological specificity. Continuous selection of the colonies of the I and the II types of fluorescence permitted to preserve the population with specific properties necessary for production of diagnostic and vaccine preparations for a long time.