The observation that radioactively labelled streptavidin binds to several biotin-containing enzymes in mammalian cells has led to the finding that there is considerable variation in the proportion of these enzymes present (namely beta-methyl crotonyl CoA; propionyl CoA; pyruvate and acetyl CoA, carboxylases). This is particularly striking when certain tumorigenic and non-tumorigenic hybrid cells are compared. It is found that there is a consistently higher proportion of pyruvate carboxylase in the tumorigenic hybrid cells. However, not all tumorigenic cell lines show this same characteristic and reasons for this are discussed. It is also shown that whilst the proportions of the four enzymes are apparently constant for a given cell type, there is a substantial degree of clonal variation and this is particularly so in tumorigenic cells in vitro. However, the more tumorigenic cells in a given population do show a higher proportion of pyruvate carboxylase. Also a range of cells derived from lymphoid tissue has been compared with normal human lymphocytes and considerable differences are again observed. The significance of these findings is considered in relation to other phenotypic properties of hybrid cells.