Crystals of hen egg-white lysozyme, grown at pH 4.7 (Alderton & Fevold 1946), are tetragonal with
a
=
b
= 79.1 Å, c = 37.9 Å, space group
P
4
3
2
1
2 (Palmer, Ballantyre & Galvin 1948; Blake, Fenn, North, Phillips & Poljak 1962). Each of the eight asymmetric units in the cell comprises a single lysozyme molecule, molecular weight about 14 600, together with 1 M sodium chloride solution which constitutes some 33.5% of the weight of the crystal (Steinrauf 1959). The structure of these crystals has been determined by X-ray analysis by the method of multiple isomorphous replacement developed in the studies of haemoglobin (Green, Ingram & Perutz 1954; Blow 1958; Perutz, Rossmann, Cullis, Muirhead, Will & North 1960) and myoglobin (Kendrew, Dickerson, Strandberg, Hart, Davies, Phillips & Shore 1960). Anomalous scattering data were used in conjunction with the isomorphous replacement intensity differences (North 1965) to form a joint probability distribution for the phase of each reflexion. The position of the centroid of each probability distribution gave a phase angle and weighting factor for each reflexion from which the electron density map with minimum r.m.s. error was calculated (Blow & Crick 1959). A large number of different heavy atom derivatives were studied (Poljak 1963; Blake, Koenig, Mair, North, Phillips & Sarma 1965) and three proved satisfactory for calculating an electron density map at 2 Å resolution. They contained respectively
ortho
-mercuri hydroxytoluene
para
-sulphonic acid, UO
2
F
5
3-
and an ion derived from UO
2
(NO
3
)
2
, probably UO
2
(OH)
n
(n-2)-
Isomorphous replacement: effects of errors on the phase probability distribution
