The paper aimed to determine the incidence of colonization of Streptococcus pyogenes, Streptococcus pneumoniae, Haemophilus influenzae type b and Neisseria meningitidis in the nasopharynges of healthy children in two preschool institutions during winter and spring months, without using antimicrobial treatment or serotyping of these bacteria. In addition to colonization of the above bacteria, the research that continued for 3 months monitored the length of their persistence in and disappearance from children's nasopharynges, children's health statuses, and provision of adequate medical interventions in children demonstrating clinical signs of disease. The ultimate aim of the paper was based on contributing to clearer and more accurate determination of a medical procedure in case of a positive result for bacteria intended to be found in the nasopharynx of a healthy child who spends time in a preschool institution.