The refined 1.5-A resolution electron density maps of both the inhibited and uninhibited forms of Staphylococcal nuclease are being fitted to model using a computer-controlled, interactive graphics system. The resultant model will allow analysis of these two forms of this enzyme at near atomic resolution. Work is also under way on mapping the active site of the nuclease using phosphonate analogues of substrate, examining complexes of the enzyme with arabinonucleotides and attempting to cyrstallize a possible transition state analogue consisting of the nuclease, VO2 ion, Ca2 ion and dooxythymidine. BIBLIOGRAPHIC REFERENCES: F.A.Cotton, R.G. Gillen, R.N. Gohil, E.E. Hazen, Jr., C.R. Kirchner, J.Nagyvary, J.P. Rouse, A.G. Stanislowski, J.D. Stevens and P.W. Tucker. Tumor-Inhibiting Properties of the Neutral P-O Ethyl Ester of Adenosine 3';3' -Mono phosphate in Correlation with its Crystal and Molecular Structure. Proc. Nat'l. Acad. Sci., 72, 1335-1339 (1975). D.M. Collins, M.D. Brice, T.F.M. LaCour and M.J. Legg. Fourier Phase Refinement and Extension by Modification of Electron Density Maps in "Crystallographic Computing II," F.R. Ahmed, ed., Munksgaard, Copenhagen, l976, in press.