AbstractIn a series of six transplantable mammary carcinomas and 11 squamous cell carcinomas of mice, evidence of tumour “progression” was obtained in the majority.Progression was evidenced by increased growth rates, transplantability and metastatic capacity, and decreased differentiation and immunogenicity (when present initially).The degree of differentiation was inversely related to the metastatic capacity of a particular tumour, and the rate of change in both parameters during serial transplantation was comparable.Treatment with retinoids in vivo and in vitro failed to stimulate differentiation of anaplastic cell lines, or to influence their growth and metastasis in the absence of host anti‐tumour immune reactivity.