Selected ion flow tube-chemical ionization mass spectrometry was used to measure formaldehyde levels in human breast cancer cells in comparison with levels in cells treated with the antitumor drugs doxorubicin (DOX) and daunorubicin (DAU) and the daunorubicin-formaldehyde conjugate Daunoform (DAUF). The measurement was performed on cell lysates and showed only background levels of formaldehyde in untreated cells and drug-treated resistant cells (MCF-7/Adr cells) but levels above background in DOX- and DAU-treated sensitive cells (MCF-7 cells). The level of formaldehyde above background was a function of drug concentration (0.5-50 microM), treatment time (3-24 h), cell density (0.3 x 10(6) to 7 x 10(6) cells/mL), and cell viability (0-100%). Higher levels of formaldehyde were observed in lysates of MCF-7 cells treated at higher drug levels, unless the treatment resulted in low cell viability. Elevated levels were directly related to cell density and were observed even with 0.5 microM drug. A lower limit for excess formaldehyde in MCF-7 cells treated with 0.5 microM DAU for 24 h is 0.3 mM. Control experiments showed that formaldehyde was not produced after cell lysis. Lysates of sensitive and resistant cells treated with 0.5 micromolar equiv of the formaldehyde conjugate (DAUF) for 3 h showed only background levels of formaldehyde. The results support a mechanism for drug cytotoxicity which involves drug induction of metabolic processes leading to formaldehyde production followed by drug utilization of formaldehyde to virtually cross-link DNA.