A rapid and highly sensitive method for the detection of formaldehyde utilizing selected ion flow tube-chemical ionization mass spectrometry is reported. Formaldehyde in aqueous biological samples is preconcentrated by distillation and directly analyzed using gas-phase thermal energy proton transfer from H30+; this procedure can be performed in 30 min. The method detection limit for formaldehyde based on seven replicate measurements of reference water samples (2.5 mL) is 80 nM at the 99% confidence level. Detection is linear up to 130 microM. This technique allows the first measurement of natural formaldehyde levels in human cancer cells in vitro. Elevated levels of formaldehyde relative to the reference water are observed for doxorubicin-sensitive cells (MCF-7 breast cancer, K562 leukemia, HeLa S3 cervical cancer) with estimated intracellular formaldehyde concentrations ranging from 1.5 to 4.0 microM, whereas formaldehyde in doxorubicin-resistant MCF-7/Adr breast cancer cells is essentially at reference level. This trend is inverted for prostate cancer cells LNCaP (sensitive) and DU-145 (resistant). Correlation of natural formaldehyde level with doxorubicin cytotoxicity is a function of the expression of enzymes that neutralize oxidative stress and the drug efflux pump, P-170 glycoprotein.