Background. The topical problems in balneotherapeutics and health resort activities in general are the development of the yellow clay-based preparations enriched in the natural mineral bischofite and the investigation of their therapeutic effects with the purpose of improvement of their curative potential. Aim. The objective of the present study was to evaluate the effectiveness of the course of the experimental therapeutic applications of the yellow clay obtained by the open pit mining of its deposits located in the vicinity of Pyatigorsk and modified by the addition of bischofite. Material and methods. The experiments were carried out with the use of 57 eugamic Wistar male rats with adjuvant arthritis. The yellow clay supplemented with bischofite in various concentrations was applied in 12 procedures each 15 minutes in duration every other or every two days beginning from the 38th day of experimentally-induced arthritis. The animals were allocated to five groups as follows: group 1 included the healthy animals that served as controls (n=5), groups 2-5 were comprised of the animals with adjuvant arthritis. The animals included in group 2 served as the experimental model of adjuvant arthritis (n=8), those comprising group 3 were treated by the applications of yellow clay without additives (n=14), group 4 was composed of the animals with adjuvant arthritis treated by the applications of yellow clay containing 10% of bischofite (n=15), the animals in group 5 having adjuvant arthritis were treated by the applications of yellow clay containing 20% of bischofite (n=15). Results. The study has demonstrated the reduction of the reactively enhanced relative abundance of, activated T-cells, and B-lymphocytes down to the respective control values in the animals with adjuvant arthritis after the course of applications of the yellow clay preparations. Simultaneously, normalization of the immunoregulatory index, IRI, (the T-helpers to cytotoxic T-lymphocytes ratio) was documented. The treatment was associated with the strong dependence of the interleukin (IL-1β) contents on the cortisol level and the well apparent relationship between the levels of interleukins and IRI. Specifically, the relation between IL-1β and IRI following the application of non-modified yellow clay was characterized as (r=+0.8, p=0.03) in comparison with the relation between IL 6 and IRI (r=+0.8, p=0.03) after the treatment with yellow clay containing 10% of bischofite or between IL 6 and IRI (r=+0.8, p=0.004) after the application of yellow clay containing 20% of bischofite. Conclusion. The results of the study give evidence of the immunomodulatory action of yellow clay modified by the addition of bischofite. They suggest the promising opportunities for the application of these preparations for the management and rehabilitation of the patients presenting with the diseases of the musculoskeletal system in the future.