TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation (Claimed): Preparation of co-crystal of lamivudine with zidovudine involves:(1) process A: trituration of equimolar quantities of lamivudine and zidovudine in the presence of water at ambient temperature, with optional addition of organic solvent during trituration; and drying of the product;(2) process B: stirring equimolar quantities of lamivudine and zidovudine in an organic solvent in the presence of water at ambient temperature; evaporation of solvents of the solution; allowing the product to get crystallized with optional seeding; and isolation of the co-crystals and drying.In process (A), the solid mixture obtained after trituration is added to the mixture of organic solvent and water, stirred at 55-65 degrees C, allowing the solvents to evaporate slowly and seeded to obtain co-crystal. Preferred Compounds: The co-crystal has peak of endotherm at about 100 degrees C in its differential scanning calorimetry profile. The co-crystal shows sharp weight loss of 1.2 to 1.9% at 80-100 degrees in its thermogravimetric analysis. The co-crystal has unit cell dimensions as given in the specification. Preferred Components: The organic solvent in process (A) is selected from 1-4C aliphatic alcohols (preferably ethanol, methanol, acetone and/or acetonitrile, especially ethanol). The organic solvent in process (B) is ethanol, methanol, acetone and/or acetonitrile. ACTIVITY - Anti-HIV. No biological data given.    MECHANISM OF ACTION - None given.    ADMINISTRATION - The pharmaceutical composition is administered orally in tablet form (claimed). No dosage details given.    EXAMPLE - Lamivudine (458 mg) and zidovudine (534 mg) were taken in a mortar and pestle and the mixture was triturated for 5 minutes to make a homogeneous mixture. Ethanol (5-6 drops) and 5-6 drops of water were added to mixture at that stage. The mixture was vigorously ground for a subsequent 15 minutes. The product obtained was further dried at 40 degrees C in vacuum drier for four hours to form co-crystal (yield: 0.992 g) having melting point of 94-95 degrees C.