Administration of cyclosporin A (CyA) to mice prior to infection and at weekly intervals during the infection with Giardia muris resulted in an increase in cyst output and a delay of the elimination phase of the infection. A short term treatment (4 days) of infected mice with CyA induced a significantly higher cyst release after treatment. CyA did not affect the ability of immune mice to resist reinfection. Our findings indicate that CyA has compromised immunological control of the primary Giardia infection, but not the ability of the immune host to resist reinfection. We propose that the use of immunosuppressive drugs may be a contributing factor in transmission of giardiasis.