Previously reported activated clotting time (ACT) data in adults demonstrated higher values with the HemoTec LRACT (HT) and TriMed ACTivator (TM) techniques than with the Hemochron System P214/215 (HC) technique throughout a range of heparin concentrations. This study sought to determine if a difference exists in ACT values of neonatal patients receiving ECMO. ACTs were performed in nine neonatal ECMO patients using the HC, HT, and TM techniques. Techniques were compared for positive or negative direction of any prediction difference (bias), and the typical value of a difference (precision). Simultaneous, duplicate, morning, and afternoon ACT comparisons were obtained using all three techniques. Forty-six comparisons of HC values in the 180-240 sec range were analyzed. All techniques produced results different from the same sample. The HT and TM techniques were upwardly biased by 51 and 148 sec, respectively, when evaluated against HC. HT was negatively biased by 123 sec when evaluated against TM. Because ACT values vary among techniques, ACT target ranges should be technique specific. Future references to ACT data should identify the equipment and procedures employed.