In this paper, we investigate the effect of P2O5 substitution by B2O3 in the (50 - x)P2O5·20CaO·20SrO·10Na2O·x B2O3 glass system (x from 0 to 5 mol%) on the thermal and structural properties and also on the glass reactivity in simulated body fluid. The goal is to develop new glass candidates for biomedical glass fibers. The addn. of B2O3 at the expense of P2O5 increases the refractive index of the glass and also the thermal stability of the glass indicating that these glasses are promising glasses for fiber drawing. Thus, within such glass compn., the core of a core-clad fiber has a larger concn. of B2O3 than the clad of the fiber to enable the light to propagate inside the core. All the investigated glasses form a calcium phosphate layer at their surface when immersed in simulated body fluid. It was found that small addn. of B2O3 (1.25 mol%) leads to a decrease in the initial dissoln. rate and a delayed layer formation. However, with increasing B2O3 content, the chem. durability decreased slightly but was higher than for the B-free glass. In addn., formation of the calcium phosphate layer was further delayed. This suggests that small contents of B2O3 led to formation of P-O-B bonds and only few BO4 units, increasing the chem. durability. At higher B2O3 contents, the amt. of BO4 units increases which makes the glass network slightly more prone to be hydrolyzed. Thus, formation of BO4 units induced by the addn. of B2O3 at the expense of P2O5 reduces the reactivity of the glass in SBF. Borophosphate fibers were successfully drawn from preform. As expected from the bioresponse of the bulk glasses in simulated body fluid, the redn. in the intensity of the light transmitted is less and slower in a borophosphate fiber than in a phosphate fiber upon immersion.