This study will compare the pharmacokinetics, safety and tolerability of two different doses of 3TC in healthy volunteers. The study will take place at Chelsea and Westminster Hospital. Twenty four healthy HIV negative volunteers will be randomly allocated into two groups. Volunteers in Group 1 will start 300mg 3TC once daily for 10 days, followed by 10 days of not taking any 3TC (wash−out period). When the wash−out period ends, they will re−start 3TC at a dose of 150mg once daily for 10 days. Group 2 is similar except that they will start 150mg 3TC at the beginning of the study and 300mg 3TC after the wash−out period. Blood samples will be taken over a 24-hour period at the end of each dosing phase to measure the levels of 3TC in the blood and inside blood cells. Safety and tolerability of 3TC will be assessed by questions, physical examination and laboratory parameters. These will be performed at regular intervals during the treatment phases.