Royal jelly (RJ) has diverse physiological and pharmacological functions. We observed its weak estrogenic activity in the previous study. RJ stimulated the proliferation of mouse osteoblast-like MC3T3-E1 cells at 0.1 mg/ml, and the effect was blocked by the specific estrogen receptor antagonist ICI 182,780. The addition of 0.1-1.0 mg/ml RJ enhanced collagen production in culture medium. Oral administration of RJ to normal female mice for 9 weeks increased the ash content of their tibiae. DNA microarray analysis revealed significant changes in gene expression related to extracellular matrix formation when the femurs of mice fed RJ were analyzed. Quantitative reverse transcriptase-PCR (RT-PCR) confirmed up-regulation of procollagen I alpha1 gene expression. These data suggest that RJ as a whole or some of its individual components stimulates production of type I collagen and other activities for bone formation through action on osteoblasts.