Vivapur® is microcrystalline cellulose manufactured by the German firm J. Rettenmeier & Söhne GmbH+Co. The types Vivapur 102 and 12 enjoy priority use as dry binders for direct tablet compression. The present paper evaluates tensile strength of tablets made from these substances and the effect of an addition of the lubricantmagnesium stearate in connectionwith its concentration and the conditions of the process of mixing, particularly the period and intensity of mixing. The tested concentrations of stearate were 0.4 and 0.8 %, the tested periods of mixing being 2.5, 5, 10, and 20 minutes, intensities ofmixing 17 and 34 rot./min. Sensitivity of dry binders to added stearate was evaluated by means of the LSR (lubricant sensitivity ratio) values. The results demonstrated higher sensitivity to an addition of the lubricant in Vivapur 12 than in Vivapur 102. In the first part of the paper focused on the effect of stearate concentration on tensile strength of tablets,Vivapur 102 was also compared with Avicel PH-102. Tablets from Vivapur 102 alone were stronger than those from Avicel PH-102. A concentration of stearate of 0.8 % decreased the binding capacity of Vivapur 102 more than that of Avicel PH-102. With a prolonged period of mixing and increased intensity of mixing with stearate, tensile strength of tablets from both Vivapur types was decresed, and a prolonged period ofmixing exerted a more marked effect onVivapur 12 and increased intensity of mixing, on Vivapur 102.

        Key words: microcrystalline cellulose –magnesium stearate – tensile strength of tablets – period and intensity of mixing