The influence of the intracellular K⁺ concentration on the effects of growth factors (insulin, EGF, hydrocortisone, and transferrin) and LDL on growth of HeLa cells was investigated. Upon replacenient of K⁺ in a chemically defined medium (K⁺‐CDM) by Rb⁺ (Rb⁺‐CDM), about 80% of the intracellular K⁺ was replaced by Rb⁺ within 24 h, but showed no further change in the next 24 h, irrespective of addition of dialyzed calf serum (5%) or growth factors to the medium. In Rb⁺‐CDM, cell growth and DNA synthesis were greatly suppressed, although cell viability was not significantly altered for 72 h. The suppression of cell growth was partially restored by addition of serum, insulin (5 μg/ml), or LDL (2.5 mg/ml) to Rb+ ‐CDM. A combination of serum and insulin or insulin and LDL stimulated cell growth to approximately the level in K⁺‐CDM without any addition, but a combination of serum and LDL did not have more effect than that of serum alone. Unexpectedly, other factors were ineffective in stimulating growth in Rb⁺‐CDM. In Rb⁺‐CDM, the effect of insulin was lost in 24–48 h, whereas that of LDL persisted for at least 96 h. Insulin and LDL also enhanced growth in K⁺‐CDM. After cessation of cell growth in Rb⁺‐CDM for 24 h, addition of insulin and/or LDL markedly restored cell growth and DNA synthesis. Therefore, insulin and LDL may stimulate certain mechanisms required for cell growth that can operate in K⁺‐deficient conditions.