A series of core mutations were introduced into beta-strand segments of an immunoglobulin fold (the isolated first domain of CD2, CD2.d1) to examine their influence on the rapidly formed intermediate state (I-state) which transiently accumulates in the folding reaction [Parker, M. J., and Clarke, A. R. (1997) Biochemistry 36, 5786-5794]. The residue changes were chemically conservative, each representing the removal of one or two methylene groups from aliphatic side chains. Predictably, the mutations destabilize the folded state with respect to the unfolded state by about 1.1 +/- 0.7 kcal mol-1 per methylene group removed. However, when the folding reaction is dissected by transient kinetic analysis into its component steps, six out of the nine mutations lead to a stabilization of the I-state. The direction and magnitude of these effects on the global stability of the transient intermediate are well correlated with changes in secondary structure propensity occasioned by the substitutions. The results show that, although side chain interactions are extremely weak in this early phase of folding, the beta-strand conformation of the polypeptide chain is established. In the next phase of the reaction, the rate-limiting transition state is attained by the formation of a tightly localized hydrophobic nucleus which includes residues V30, I18, and V78. Interestingly, in almost all immunoglobulin domains of extracellular proteins, the latter pair are cysteine residues which form a disulfide bridge.