An organ-specific tumor-associated antigen (TAA) was present in several metastatic and nonmetastatic mammary carcinomas induced in WF female rats by 3-methylcholanthrene. The level of TAA was high in 2 metastatic carcinomas tested (TMT-081 and SMT-2A) and much lower--by a factor of 50--200--in 2 nonmetastatic mammary carcinomas (MT-100 and MT-W9B). The TAA in the 2 metastatic tumors was identical, as demonstrated by immunodiffusion and supported by cross-reactivity with antibody against TAA from TMT-081 in a binding inhibition radioimmunoassay. The TAA was shed in relatively large amounts by the metastatic tumors maintained in short-term organ culture. The high level and shedding of TAA thus appeared to be characteristics of the metastatic tumors but not of the nonmetastatic ones. This suggests that TAA on the cell membrane or in the circulation may be involved in the metastatic process as a factor blocking potentially cytotoxic cells or in other ways leading to suppression of the immune response against the tumor.