The course of Giardia lamblia infection in primary and secondary infection and early and late treated Swiss albino mice was studied. Animals treated after 3 days of infection did not show any resistance to a second challenge, whereas those treated after 9 days did show resistance to reinfection. Reinfected animals eliminated the parasites faster than the other groups. The maximum antibody levels in primary, reinfected, early and late treated groups were observed on the 20th, 10th, 20th and 15th post infection days respectively.