It is known that the essential function of extracellular HSP70 (e-HSP70) to protect cell processes, indirectly associated with the genetic structures. A direct influence of e-HSP70 on chromosomal stability has not been studied. This explains actuality of the suggested investigation. A study was made of the influence of e-HSP70 on chromosomal variability at different phases of the first mitotic cycle in both intact and ciprofloxacin (CF) treated cells of the Indian muntjac skin fibroblasts. E-HSP70 (10 mg/ml) exerts no influence on the level of chromosomal aberrations, typical of the control. After a joint action of e-HSP70 and CF (50 mg/ml) no influence was also exerted on the antibiotic induced genotoxicity effect. CF and e-HSP70 (50 and 100 mg/ml, resp.) acting apart on intact cells during 6 and 24 h induce a significant increase of chromosomal aberrations compared to the control, primarily at the expense of chromatid or chromosomal breaks (depending on the duration of respective effect). CF and e-HSP70 acting apart on intact cells during 6 h with the following cultivation for 18 h in the fresh medium prior to fixation also induce a significant increase of chromosomal aberrations compared to the control, primarily at the expense of both breaks and dicentrics (telomeric associations). The joint action of CF and e-HSP70 on cultivated cells during 6 and 24 h and when CF and e-HSP70 were added respectively on 24 and 6 h prior to fixation, a significant decrease in chromosomal aberrations compared to the control level was induced. A simultaneous addition of CF and e-HSP70 in 6 h with the following cultivation for 18 h in fresh medium prior to fixation exerted no influence on the degree of genotoxicity effect, typical of the separate action of these agents. However a significant decrease in the number of dicentrics occurred. Apparently, e-HSP70 has a protective effect on chromosomal stability mainly at phase 62 of the mitotic cycle. The denaturated e-HSP70 (50, 100 mg/ml) has a genotoxicity effect similar to that of the infact e-HSP70 under above conditions. The joint action of CF and denaturated e-HSP70 (50 mg/ml) during 24 h exerts no influence on the degree of genotoxicity effect, induced by the separate action of these agents or leads to an increase in the number of dicentrics, acting separately or jointly compared to the control. The denaturated e-HSP70 (100 mg/ml) acting jointly with CF for 24 h, increases the degree of genotoxicity effects, induced by separate actions of the agents. Lipopolysacharide (50 mg/ml) exerts no influence on the number of chromosomal aberrations in the control. The sensitivity of individual chromosomes and their regions to agents inducing chromosomal instability is different. The preferential involvement of some chromosomes in dicentric formation was observed. A possible role of dicentrics in adaptation of cells belonging to "markerless" lines to infavourable factors of the environment, and possible mechanisms of protecting effect exerted by e-HSP70 on chromosomes are discussed.