A modified biomaterial of periwinkle shell powder, MBPSP derivative of Tympanotonus fuscata shell was applied as a diluent in theophylline hydrate (TPH) and hydrochlorothiazide (HCTZ) tablets. A 100 g of the pulverized periwinkle shell was digested with 166.0 ml of 2 M hydrochloric acid, filtered and neutralized with 5 M sodium hydroxide. Ortho-phosphoric acid was added drop wise until a thick, dehydrated mass was obtained. It was dried at 60 o C and milled to 250 I¼m size and designated as MBPSP. Using this, batches of granules containing TPH (100 mg) and HCTZ (50 mg) were produced by the wet granulation technique alongside lactose and dicalcium phosphate dihydrate (DCP). They were tableted at 7.55kN using a 10.50 mm diameter die and flat-faced punch on a hydraulic hand press. Tablet properties were evaluated. The weight variation for the TPH or HCTZ tablets produced from MBPSP, lactose or DCP were between 0.03- 0.32 %. The total drug content for the respective batches of tablets was within 85-118 % of the claimed label potency. Though the respective batches of tablets of TPH and HCTZ containing MBPSP, lactose or DCP maintained significantly high values of mechanical strength, the tablets of TPH and HCTZ prepared with DCP generally showed significantly higher values of mechanical strength than those of MBPSP and lactose. Dissolution profile showed that at 45.00 min, 73.13, 87.72 and 87.45 % of TPH was released from the tablets containing lactose, MBPSP and DCP respectively while at 60.00 min, 64.58, 57.84 and 66.00 % of HCTZ was dissolved from the tablets having lactose, MBPSP and DCP respectively. Though MBPSP compared significantly favourably with lactose and DCP in the various tablet parameters evaluated, the tablets of TPH or HCTZ comprising MBPSP displayed very significant shorter tablet disintegration time and also maintained a significantly greater mechanical strength in comparison to lactose and DCP. Therefore, MBPSP could be a very useful excipient in the formulation of TPH and HCTZ immediate-release tablets. Keywords: Modified biomaterial, Tympanotonus fuscata, periwinkle, shell, theophylline hydrate, hydrochlorothiazide, tablet.Â