Objective To observe the effect of seropharmacological plastrum testudinis on the expression of bone morphogenetic protein4 (BMP4) of rat mesenchymal stem cells (MSCs) in vitro. Methods Mesenchymal stem cells were dissociated from rat bone marrow and were marked by Brdu and the expression of CD44. The growth of rat mesenchymal stem cells with low, middle and high seropharmacological plastrum testudinis were observed separately by means of morphological apperances. The locations of the BMP4 protein and BMP4 mRNA positive cells were detested by immunohistochemistry and fluorescent immunohistochemistry combined with laser confocal scanning microscopy and in situ hybridization stained by DAB. The expression quantity of BMP4 mRNA and BMP4 protein was detected by real time PCR and ELISA. Results Seropharmacological plastrum testudinis promoted the numbers of BMP4 and BMP4 mRNA positive cells and the expressions of BMP4 protein and BMP4 mRNA in a time and dosedependant manner; there were significant differences in comparison with control groups. Conclusion Promoting effect of seropharmacological plastrum testudinis on the prolife