Toughness-curve (T-curve) behavior of composites of spherical, polgcrystalline, coarse-grained, alumina agglomerates dispersed throughout a constant-toughness, fine-grained, 50-50 vol% alumina-mullite matrix has been evaluated as a function of agglomerate content for the range 15 to 45 vol%. T-curve behavior was evaluated using the indentation-strength method, Increasing alumina agglomerate content resulted in a progressive increase of large indentation load strengths with negligible change of plateau strength levels at small indentation loads, This behavior is consistent with underlying T-curves that rise to greater values and are shifted toward longer crack lengths with increasing agglomerate content, suggesting that both bridge spacing and bridge potency increase with increasing agglomerate content over the range tested. The proposed relationships between bridge spacing and agglomerate content, and bridge potency and agglomerate content, are rationalized in terms of residual stress considerations. The indentation-strength data also demonstrated that the composite containing the greatest alumina agglomerate content, 45 vol%, exhibited the greatest flaw tolerance.