The viability of human corneal keratocytes was assessed during four weeks of 'closed system' organ culture at 31 degrees C. After 28 days of culturing, the entire keratocyte population was still alive and viable because all cells incorporated uridine; a parameter for RNA-synthesis. During the first 14 days, mitoses were found in the anterior half of the stroma (0.23% mitoses per 48 h), while only few keratocytes were able to divide at day 28 (0.01% mitoses per 48 h). Metabolic parameters revealed a progressing acidosis in the medium with oxygen and glucose depletion. Immunological measurements of keratan sulphate proteoglycan suggested that approximately 1% of the total content was lost during the period. In conclusion, our current organ culture technique can maintain a viable keratocyte population for four weeks; a viable stroma can be grafted within this period.