In order to examine their effects on the P-glycoprotein (P-gp) activity in human breast cancer cells, MCF-7/ADR, one hundred Indonesian plant extracts were screened. Among them, the five chloroform extracts of Calotropis gigantea, Curcuma aeruginosa, Merremia mammosa, Sindora sumatrana, and Zingiber cassumunar, showed the most potent P-gp inhibitory activity. When each of these extracts was treated together with the anticancer agent, daunomycin, they increased the cytotoxic activity of daunomycin up to $IC_{50}$ values of less than $6.62;{mu}M$, which is a value with a positive control, verapamil. Also, other 15 plant extracts exhibited significant P-gp inhibitory activity with $IC_{50}$ values between 6.62 and $13.20;{mu}M$. These prospective samples will be subjected to further laboratory phytochemical investigation to find active principles.