To determine whether anti-transferrin (Tf) receptor monoclonal antibodies might be useful in treatment of human solid tumors, in vitro effects of immunoglobulin A (42/6) and immunoglobulin G (B3/25) anti-Tf receptor antibodies on human solid tumor growth were examined. In colony and liquid cultures containing 10% serum, B3/25 did not inhibit growth of melanoma or ovarian carcinoma cell lines. 42/6 caused modest dose-dependent inhibition in colony cultures (maximum inhibition approximately 50%), and slowed growth of melanoma, ovarian carcinoma and epidermoid carcinoma cells in liquid culture. Inhibition was more pronounced in low (1%) serum, and was abrogated by 200 micrograms/ml iron-saturated Tf or 50 microM ferric nitriloacetate. All cells displayed high affinity Tf receptors (4-20 X 10(4)/cell). Cells grown in 1% serum and epidermoid carcinoma cells displayed more receptors, and susceptibility to 42/6 inhibition appeared related to higher receptor number. After culture with anti-Tf receptor antibodies, solid tumor cells showed a 57-93% reduction in surface Tf-binding sites. Tf uptake by cells grown for 24 h in B3/25 was approximately 50% of control, but was reduced to less than 10% of control with 42/6. Immunofluorescence staining of melanoma and HL60 promyelocytic leukemia cells suggested greater heterogeneity of Tf receptor display on melanoma than on leukemia cells. Previous studies showed 42/6 completely blocked blood cell Tf internalization and is a potent inhibitor of hemopoietic cell growth. In contrast, in solid tumor cells, inhibition of Tf uptake and growth inhibition are subtotal. Solid tumor resistance to 42/6 may be due in part to greater heterogeneity of Tf receptor display by proliferating cells. However, responses to iron-saturated Tf and ferric nitriloacetate in the presence of 42/6 also differ in hemopoietic and solid tumor cells, suggesting possible differences in Tf processing or iron growth requirements.