For the aim of contrasting the effect of crude yeast extraction (CYE) with that of its polysaccharide fraction (YEPF) on growth, seed yield, silymarin content and leaf anatomy of Silybum marianum plants, CYE or YEPF was applied twice as a foliar spray at either 0 (as control), 3 or 6 gL^-1. Results indicated that CYE was more effective on plant growth whereas YEPF was more beneficial for the plant's reproductive processes, resulting in higher seed yield per plant compared with CYE. Moreover, though both CYE and YEPF induced the accumulation of silymarin in plant organs, higher silymarin content in seeds, capitula, leaves and stem was recorded in response to the treatment with YEPF. In addition, the application of either CYE or YEPF at 6 gL^-1, preferabaly with the latter enhanced the photosynthetic and translocative components of the leaf tissues. The mechanism through which either CYE  or YEPF affect plant growth, seed yield and silymarin accumulation was discussed. It could be concluded that utilization of yeast extract as an in vivo elicitor is feasible and that YEPF is preferred over CYE for enhancing seed yield and silymarin content in S. marianum plants.