The methanolic extract from the whole plants of Anastatica hierochuntica, an Egyptian herbal medicine, was found to inhibit melanogenesis in theophylline-stimulated murine B16 melanoma 4A5 cells. Among the constituents isolated, anastatin A, silybin A, isosilybins A and B, eriodictyol, luteolin, kaempferol, quercetin, hierochins A and B, (2R,3S)-2,3-dihydro-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-5-(2-formylvinyl)-7-hydroxybenzofuran, (+)-dehydrodiconiferyl alcohol, (+)-balanophonin, 1-(4-hydroxy-3-methoxyphenyl)-2-{4-[(E)-3-hydroxy-1-propenyl]-2-methoxyphenoxy}-1,3-propanediol, and 3,4-dihydroxybenzaldehyde substantially inhibited melanogenesis with IC(50) values of 6.1-32 microM. With regard to the mechanism of action of silybins and isosilybins, the inhibition of tyrosinase activity suggested to be important. In addition, isosilybins A and B inhibited the mRNA expression of TRP-2, but silybins A and B oppositely enhanced the mRNA expression of tyrosinase and TRP-1 and -2 at 10 and/or 30 microM, and the inhibition of phosphorylation of extracellular signal-regulated kinases (ERK1/2) is involved in the enhanced expression of mRNA, at least in part, similar to that of PD98059.