A single Cyclosporin A (CsA) dose of 30 mg kg-1 given orally at day 4 post-infection (p.i.) to Sprague-Dawley rats infected with Strongyloides ratti, reduced the faecal larval count by 46.8 +/- 1.2%. CsA was equally effective when the same dose rate was administered subcutaneously at day 4 p.i., reducing the faecal larval count by 41.6 +/- 8.6%. Thiabendazole (TBZ) given orally at 5 or 10 mg kg-1 (single dose at day 4 p.i.) reduced the faecal larval counts by 57.1 +/- 4.1% and 69.0 +/- 9.6%, respectively. Orally administered CsA was less effective than 5 mg TBZ kg-1 (at day 4 p.i.) Co-administration of 5 mg TBZ kg-1 and CsA did not elicit synergy or additive efficacy, indicating that CsA did not antagonise the anti-strongyloides activity of TBZ. The data suggests that for patients with current, historical or serological evidence of strongyloidiasis, CsA may be used where immunosuppressive therapy is required for other concurrent reasons or when TBZ is contraindicated.