The expression of a melanoma-associated antigen, recognized by the monoclonal antibody 9.2.27, has been studied in the human FME melanoma cell line, grown as a monolayer under various conditions in vitro and as tumours in athymic mice. Two-parameter flow cytometric measurements of DNA and immunofluorescence showed that the antigen expression was uniform throughout the cell cycle. Highest expression of the melanoma-associated antigen was found in cultures in which the medium was frequently renewed (4 times in 6 days) and no contact inhibition was present. In comparison, the antigen expression of cells subjected to medium starvation (6 days without medium renewal) was reduced to 44%, and the cell size, as measured by forward angle light scatter, to 82% of the values found for optimally growing cells. Intermediate conditions, such as scanty medium supply or contact inhibition of growth, gave smaller effects. Two-parameter measurements of cell size and immunofluorescence demonstrated a positive correlation between cell size and antigen expression. The reduction in cell size, therefore, could explain part, but not all, of the reduction in antigen expression on medium-starved and contact-inhibited cells. The antigen expression on FME cells grown as xenografts in athymic mice varied between individual tumours and was lower than on cells from optimal in vitro growth. Due to smaller cell size, the antigen density was, however, comparable to that on cells from optimal in vitro growth.