The histogenetic behavior of two human cervical squamous carcinoma cell lines (C-4 I and C-4 II), originating from the same biopsy specimen and probably from the same cell clone, were examined histochemically and electron microscopically in vitro and in vivo. In culture, C-4 I cells formed compact, highly stratified, epithelial colonies, flattened on contact with culture medium, and retracted from substrata in response to crowding. In culture, C-4 II cells formed spreading, predominantly monolayered, epithelial colonies with hemicysts, joined by junctional complexes on contact with culture medium, and segregated in response to crowding. When inoculated into hamster cheek pouches, C-4 I cells grew as expanding, compact masses, whereas C-4 II cells in small groups infiltrated connective tissues. Thus the in vitro growth patterns of the cell lines were reproduced in the patterns of invasion in vivo. Glycogen was produced in vivo by C-4 I cells only, and basement membranes were found around tumors of both cell lines. The morphologic and functional characteristics of the cell lines could be interpreted as due partly to the retention of tissue-specific properties of normal epithelial cells at various levels of squamous differentiation.