Treatment with doxorubicin is associated with decreased levels of expression of muscle-specific genes in myocytes. This may be related to an effect on expression of helix-loop-helix (h-l-h) regulatory molecules since in myoblastic cells, doxorubicin inhibits Myo D expression and enhances Id expression. We have reported that expression of Id and mouse Twist (mTwi), another h-l-h molecule, decline in association with differentiation in osteoblastic cells. We have sought, therefore, to determine the effect of doxorubicin on MC-3T3-E1 osteoblastic cells. Treatment with doxorubicin decreased total cellular protein content, reduced [3H]-leucine incorporation into protein, inhibited proliferation and diminished alkaline phosphatase activity. Glucose utilization and lactate production were not adversely affected. Id expression was increased by doxorubicin treatment under growth conditions but not differentiation conditions. Expression of mTwi was markedly increased under both growth and differentiation conditions. These data support the contention that Id and mTwi may regulate differentiation in osteoblastic cells.