This study was supported by a grant from the DHSS, No. R/E 1050/105. The authors are grateful to Mrs J. Edwards who maintained the mouse colony, to Dr Buchan, Department of Medical Microbiology, for the initial supply of cells, and to Dr Riches, Department of Immunology, for use of the automated continuous flow apparatus for enzyme analysis. In addition, the help of the Photographic Unit in preparing figures is acknowledged, and grateful thanks are extended to Professor R. M. Browne for all his help and advice.