The purpose of the study was to investigate the morphological properties of oral fluid in dynamics in the application of the developed set of measures for the prevention of fissure caries of the first permanent molars in children. Materials and methods. 62 children aged 6 years with fissure caries of the first permanent molars (main group) and 31 children without signs of carious lesions (control group) were examined. The study of the crystal formation of oral fluid was carried out according to the method of Leus P. A. (1977). The mineralizing potential of oral fluid was expressed as an average score depending on the types of crystals detected. Results and discussion. The analysis of the results of the study showed that 6 months after the start of preventive measures in the oral fluid of the children of the main group, a slight increase in type I crystals was noted in comparison with the initial data (40.18 ± 4.65% of cases, p>0.5), at the same time, the number of children, in whom type III crystals were observed, decreased in the main group (16.45 ± 2.32%, p>0.5), and increased in the control group (19.32 ± 2.47% of cases, p>0.5) in comparison with the initial data. After two years of observation, in the children of the main group, type I crystals were most often detected in the oral fluid (48.41 ± 4.23% of cases), in the children of the control group, on the contrary, the number of children with this type of crystals decreased and was almost one and a half times less than among children of the main group (p<0.05). On the other hand, the number of children in the control group, in whom type III crystals were detected, was two times higher than among children in the main group (22.34 ± 2.87% and 11.51 ± 2.56% of cases, respectively, p<0.05). Conclusion. It was established that in children who received a complex of preventive measures, 6 months after the implementation of preventive measures, the mineralizing potential of oral fluid was, on average, 2.73 ± 0.07 points, which is slightly higher compared to the fissure detected at the beginning of caries prevention (2.57 ± 0.12 points respectively, p>0.05). In the children of the control group, the mineralizing potential of oral fluid changed less noticeably (from 2.64 ± 0.11 points at the beginning of observation to 2.66 ± 0.11 points, p>0.5). After two years of observation, it was found that in the main group the mineralizing potential of oral fluid continued to increase, while in the children of the control group it decreased and reached almost the initial level (2.65 ± 0.07 points in the second year of observation and 2.64 ± 0.08 points at the beginning, p>0.05). Thus, it can be concluded that the proposed measures are quite effective