Pieces of human psoriatic skin and suspensions of mouse bone marrow cells as non-epidermal control tissue were cultured for four days in diffusion chambers implanted intraperitoneally in 40 mice. The mice were divided into three groups. On the second and third day of the culture partially purified epidermal extract containing epidermal chalone, granulocyte extract containing granulocyte chalone, and Hanks' solution were injected intraperitoneally into the mice of the three groups, respectively. On the fourth day, incorporation of 3H-TdR into the diffusion chamber cultures and various epithelia and several non-epidermal tissues of the host animal was studied autoradiographically and radiochemically. The results showed that the epidermal chalone induced a marked, statistically significant, depression in the labelling index of the cultured psoriatic epidermal cells and in the number of labelled cells in the host ear epidermis (31% and 62%, respectively); the other host epithelia were not significantly inhibited. A crude granulocyte extract used in most experiments was clearly toxic; the semipurified granulocyte chalone used in one experiment had no effect on the psoriatic and cultures, but affected weakly the cultured bone marrow cells. Neither the epidermal nor the granulocyte extract had any inhibitory effect on the non-epidermal tissues studied.