ABO blood type has been shown to be associated with both cardiovascular risk and toxicity-preventing activity (TxPA) stress response in elderly males. Twenty middle-aged, healthy males, 14 blood type A and six blood type O, were involved in this project. Volunteers completed a battery of psychological assessments, then gave blood and had several psychophysiological measures taken prior to, during and after two stressors. The stressors consisted of mental arithmetic tasks plus audiotapes of combat sounds and a baby crying. The anger-out and hard-driving scores of blood type O subjects were significantly higher than the blood type A means. TxPA decreased significantly as a function of stress and some suggestive blood type effects of TxPA were found. Plasma protein, microhematocrit, plasma cortisol, finger temperature, skin conductance, blood pressure and two facial electromyograph (EMG) variables were also significantly affected by stressors but not by the blood type factor. No significant differences of any kind were found for total cholesterol, high-density lipoprotein or pulse variables. The importance of age and other individual subject characteristics was discussed.