We examined the food additive, butylated hydroxyanisole (BHA), for its capacity to modulate the cytotoxic effects of Delta(9)-tetrahydrocannabinol (THC). THC was not cytotoxic when added to cultures of A549 lung tumor cells at concentrations<5 microg/ml, but induced cell necrosis at higher levels with an LC(50)=16-18 microg/ml. BHA alone, at concentrations of 10-200 microM, produced limited cell toxicity but significantly enhanced the necrotic death resulting from concurrent exposure to THC. In the presence of BHA at 200 microM, the LC(50) for THC decreased to 10-12 microg/ml. Similar results were obtained with smoke extracts prepared from marijuana cigarettes, but not with extracts from tobacco or placebo marijuana cigarettes (containing no THC). Two different mechanisms for this synergistic cytotoxicity were investigated. Experiments were repeated in the presence of either diphenyleneiodonium or dicumarol as inhibitors of the redox cycling pathway. Neither of these compounds protected cells from the effects of combined THC and BHA, but rather enhanced necrotic cell death. Measurements of cellular ATP revealed that both THC and BHA reduced ATP levels in A549 cells, consistent with toxic effects on mitochondrial electron transport. The combination was synergistic in this respect, reducing ATP levels to <15% of control. Exposure to marijuana smoke in conjunction with BHA, a common food additive, may promote deleterious health effects in the lung.