The present study was designed in order to examine the objectivity and reliability of the individual anaerobic threshold (IAT) as well as its resistance against several interfering factors: missing exhaustion, preliminary exercise, longer step duration, and lower speed increment. 87 male and 24 female runners and triathletes were examined. They performed both the original test procedure (IAT(3/2); 3 min step duration, 2 km x h(-1) increment until volitional exhaustion) and either a retest or one of several alternative test procedures (submaximal, preliminary exercise, 5 min step duration, 1 km x h(-1) increment) to be compared with IAT(3/2). The graphic determination of the IAT is characterized by a low inter-observer-variability without significant differences between 4 independent blinded examiners. Both the lactate performance curve (p = 0.07) as well as the heart rate performance curve (p = 0.05) show a slight shift to higher velocities during the retest. The IAT shows an identical running velocity during the retest, the heart rate tends to be lower, and the lactate concentration is significantly (by 0.26 mmol x l(-1); p < 0.05) lower than during the first test. Both a low degree of exhaustion (3-6 mmol x l(-1) lactate; IAT(3/2) shortened by 180 s) and an extension of the step duration from 3 to 5 min do not lead to significantly different velocities at the IAT. Moderate preliminary exercises (approximately 4 mmol x l(-1) lactate) do not influence the velocity at the IAT, the heart rate is significantly higher. A reduction of the speed increment from 2 to 1 km x h(-1) significantly increases the velocity at the IAT by 6%. It is concluded that the determination of the IAT is highly objective, reliable and insensitive to changes of the incremental graded testing protocol, such as a previous warm-up, an extension of the step duration from 3 to 5 min as well as a lower degree of exhaustion. Significant differences may only arise from changes in the speed increment.