The presence of contractile proteins in human cancer cells has been studied by means of: a) immunofluorescent staining using specific antibodies, and b) electron microscopy in order to detect the presence of cytoplasmic filaments. The tissues examined were: normal human skin, basal cell carcinoma of the skin, squamous cell carcinomas (of skin, oral cavity, and larynx), normal nonlactating mammary gland, and infiltrating mammary carcinoma with or without fibrosis. Normal tissues were negative after immunnoflurosescent staining of contractile proteins and contained no or minimal amounts of microfilaments as judged by electron microscopy. Tumor cells were strongly positive after immunoflouorescent staining for actin, myosin, light and heavy meromyosin but were negative for tropomyosin. Moreover, they contained prominent microfilaments (40 to 80 A in diameter) with some filaments (100 to 120 A in diameter) scattered in between. It appears that malignant cells contain an increased amount of contractile proteins, organized in the form of a filamentous apparatus, when compared to their normal counterparts. The study of the presence of contractile proteins in tumor cells may be of potential importance in evaluating malignant growth.