The phosphonylmethoxyalkyl derivative, (S)-1-(3-hydroxy-2-phosphonyl methoxypropyl)cytosine (HPMPC), was evaluated for its efficacy in the topical treatment of experimental keratitis caused by thymidine kinase-positive (TK+) or thymidine kinase-deficient (TK-) herpes simplex virus type 1 (HSV-1) strains. The HPMPC 0.2% eyedrops were as effective as the reference compound, (E)-5-(2-bromovinyl)-2'-deoxyuridine, (BVDU) 0.2% eyedrops in stimulating the healing of epithelial disease caused by the HSV-1 TK+ strain. Both drugs achieved a significant (P less than 0.005) healing effect compared with placebo eyedrops. No significant differences were noted in the efficacy of HPMPC 0.2% eyedrops when instilled one, three, or nine times a day. In the treatment of keratitis caused by the HSV-1 TK- strain, 0.2% BVDU eyedrops were similar to placebo; 0.2% HPMPC eyedrops again had a brisk and significant healing effect (P less than 0.005).