The effect of low concentrations of acetaldehyde on activated partial thromboplastin time (APTT) and prothrombin time (PT) of Accuclot coagulation plasmas was monitored over a prolonged time to mimic effects observed in alcoholism. A prolongation of the APTT from 31.9 +/- 0.7 s to 32.6 +/- 0.9 s (n = 8; P =.007) was observed after a 30-min preincubation time with 140 microM acetaldehyde. However, a minimum of 3.6 mM acetaldehyde was required to extend the APTT from 36.6 +/- 1.0 s to 41.2 +/- 0.8 s (P =.001) over an 18-h exposure time. Plasma acetaldehyde levels as low as 2.24 mM caused elevation of PTs from 12.5 +/- 0.5 s to 14.4 +/- 0.2 s (P =.005) after a 24-h preincubation time. These findings seem to indicate that short-term contact of acetaldehyde with plasma, probably yielding reversible interactions, may interfere with APTTs to a greater extent than long-term contact, which would presumably yield stable, irreversible interactions. In comparing the effects of 8.94, 17.9, 89.4, and 447 mM acetaldehyde on the PTs of Level I, II, and III plasma, the PTs were most increasingly prolonged in Level III plasma and least prolonged in Level I plasma at each acetaldehyde concentration, although the plasmas have comparable protein concentrations. These findings seem to indicate that coagulation factors are sensitive to inactivation by acetaldehyde.