Mersilene (polyester monofilament) seems to be suitable for penetrating keratoplasty because it is strong, shows no degradation by ultraviolet light, is insoluble, so that it can be left in situ, and offers the possibility of regulating postoperative astigmatism by suture adjustment. In 12 patients penetrating keratoplasty was performed with the combined interrupted/running suturing technique, using eight interrupted nylon 10-0 sutures and one running Mersilene 11-0 suture. The results were compared with those of 25 patients in whom eight interrupted nylon 10-0 sutures and one running nylon 11-0 suture were used. Six months after penetrating keratoplasty, no differences could be found between the two groups in keratometric astigmatism, visual acuity or slitlamp findings. In three patients postoperative adjustment of the running Mersilene suture reduced astigmatism by 50, 90 and 100% respectively. In an animal study the behaviour of Mersilene in the cornea was evaluated by slitlamp examination, histology and electron-microscopy. The tissue response to Mersilene was minimal. Considering the resemblance to nylon in clinical findings, minimal tissue response, lack of biodegradation and possibility of regulating postoperative astigmatism by suture adjustment, Mersilene seems to be a suitable material for penetrating keratoplasty.