This study was supported in part by a grant from the DHSS, No. R/E 1050/105, and the Amalgamated Dental Company. The authors are grateful to Mrs G. Smith, Mr P. Stephens and Mrs G. Bowen for skilled technical assistance; to Dr Buchan, Department of Medical Microbiology, for the initial supply of fibroblasts and to Dr Perris, Department of Biological Sciences, University of Aston, for the use of the Coulter counter. In addition, the help of the Photographic Unit in preparing figures is acknowledged.