Abstract The mouse skin bioassay has been used to evaluate tumorgenicity of various agents for many years This presentations will include data compiled over the last years in the Kettering Laboratory All bioassays reported here wee done on male C3H mice In the review a variety of dermal and epidermal neoplasms was encountered Some carcinomas of the epidermal develop from benign tumors or directly@ while other were multifocal We will discuss the influence of the vehicle (solvent) on the type of neoplasm. Carcinomas or sarcomas@ that develop Incidence and latent period will be presented on spontaneous tumors in treatment mice Additionally@ occurrence and types of neoplasm in animals treated with various vehicles will be discussed Also included will be the incidence of neoplasms in positive controls treated with different concentrations of benzo(a)pyrene Similar data will be presented on various test materials such as crude petroleum fractions of petroleum. Shale oil@ coal tar@ and asphalts The skin painting braossay performed on mice developed from investigations to experimentally produce neoplasms in animals. It has been used over the years to elucidate the mechanisms of carcinogenesis Most of the skin painting with chemicals is done today to evaluate carcinogenesis potentials of these materials Frequently we want to know how one chemical compares with another@ or with positive standards In other words@ is the carcinogenesis potency Just the number of neoplasms caused by the applied material does not give the answer The knowledge of the natural history of experimental tumors is of great importance When an oncologist or pathologist is confronted with a neoplasm certain basic questions are asked The answers to these are a summary of our knowledge of the behavior of a specific neoplasm. Or in other words@ its natural history First we ask what type of neoplasm is present and where is the site of origin. Since the most frequent experimental neoplasm is the squamous cell carcinoma@ the presentation will be limited to this tumor Therefore@ the cell type is the squamous cell and the site is the skin of the interscapular Is this important Yes In humans we grade the tumor according to its cytology In the case whereby most of the cells are well differentiated and only the minor percentage show malignant features (75% normal. 25% atypical)@ the neoplasm is Grade 1 Grade 4 represents an anaplastic tumor@ or all cells show severe features of malignancy A similarity exists in experimental squamous cell carcinomas In Figure 1 given Figure 3 represents a squamous cell carcinoma of unique type Besides the epithelial components@ there is a large amount of stroma or fibrous tissue present The stroma shows no malignant features Personally@ I have not seen this neoplasm in humans Depending on the grading and the inventiveness of the tumor@ the probability of metastases is less and more likely