Theoretical expressions for the probability of success of the quasi-anomalous method have been derived for triclinic, monoclinic and orthorhombic crystals containing a few (1, 2 or 3) heavy atoms per asymmetric unit besides a large number of light atoms. The results derived take into account data-truncation due to unobserved reflections. Using the theoretical expressions, tables of probability values for the success of the quasi-anomalous method are obtained as a function of the relevant parametersk andδ₁². Corresponding results for triclinic crystals containing many heavy atoms (i.e.P = MN and MC cases) have also been obtained. It is seen that, using suitable heavy atoms to prepare the heavy-atom derivative, probability of success as high as 0.7 could be obtained in the case of proteins containing 1000 to 1500 atoms.