Transformed baby-hamster-kidney cells contain higher intracellular concentrations of polyamines than do normal cells. The difference is greater in high-density confluent cultures. Transformed cells incorporate exogenous putrescine into the cells at a faster rate than do normal cells. They also show a marked increase in the rate of spermine biosynthesis compared with normal cells. Transformed cells grown to high cell densities released about 10% of their polyamines into the culture medium in a non-specific manner. In contrast, normal cells, under the same culture conditions, release up to 50% of their intracellular polyamines into the medium almost exclusively as free or conjugated spermidine. The elevated levels of polyamines found in transformed cells therefore appear to be the result of altered transport of polyamines across the cell membrane and of increased rates of biosynthesis.