A nontransformed and a spontaneously transformed clone of BALB/c 3T3 cells were compared for their capacity to multiply in decreased concentrations of Mg(2+). Cells of the nontransformed clone were flat, formed regularly patterned, nonoverlapping arrays, required high serum concentration for multiplication, had a low saturation density, and did not make colonies in agar. Cells of the transformed clone were slender and spiky, formed random, overlapping arrays, multiplied in low serum concentrations, and had no fixed saturation density, and 20-30% of them formed colonies in agar. The saturation density of the nontransformed clone was decreased in a growth-limiting supply of Mg(2+) in proportion to the reduction in initial rate of multiplication. At very low Mg(2+) concentrations, saturation occurred when less than half of the surface of the dish was covered with cells. The transformed cells did not reach a stable saturation density in low Mg(2+) concentrations, but their growth rate did slow down when they became crowded, and a transient saturation density was reached at the lowest Mg(2+) concentrations that allowed multiplication. Limiting the supply of Mg(2+) caused the transformed cells to flatten and to assume a regularly patterned, non-overlapping relationship to one another, resembling that of the nontransformed cells. This also occurred in BALB/c 3T3 cells transformed by infection with Moloney mouse sarcoma virus. After 1 week in low concentrations of Mg(2+), the nontransformed cells began to multiply and to incorporate [(3)H]thymidine at a rapid rate. The transformed cells did so also and, in addition, reverted to their transformed appearance. The intracellular content of Mg(2+) was not significantly decreased when the extracellular concentration was decreased to 1/50th. The results suggest that: (a) limited contact among cells already multiplying at a reduced rate is sufficient to halt further multiplication; (b) a very small decrease in intracellular Mg(2+) content or in membrane-associated Mg(2+) causes transformed cells to assume aspects of the appearance and behavior of nontransformed cells (i.e., Mg(2+)-regulated reactions may be involved in determining the transformed phenotype); and (c) cells multiplying at a slow rate in low concentrations of Mg(2+) begin to multiply faster after about 1 week, due either to an adaptation of the cells or to a change in the cellular microenvironment.