We evaluated a new dry-reagent carrier system for the determination of creatine kinase (EC 2.7.3.2) activity, Reflotron CK, with special attention to analytical performance with whole blood. We found a good within series imprecision. The median coefficient of variation was 3.1% for Reflotron CK (blood, serum and plasma) and 0.9% for the automatic analysers (serum and plasma only). The between-days imprecision with Reflotron CK (median CV: less than or equal to 3%) was similar to that for the comparison method on different analysers. Fresh samples of human blood, plasma and serum were examined by Reflotron CK and by a N-acetylcysteine activated creatine kinase method in six different clinical laboratories and in the Evaluation Department of Boehringer Mannheim GmbH. The correlation between these methods was excellent (r greater than or equal to 0.99), the median systematic deviation (bias) for all samples being smaller than -5%. Haematocrits between 0.25 and 0.50, haemolysis up to 6 g/l haemoglobin, and icteric samples with bilirubin concentrations up to 0.2 g/l showed no interference. No drug in therapeutic concentration was found to affect the Reflotron CK results; ascorbic acid, calcium dobesilate and sulphamethoxazole lowered the values only when present in high concentrations. Reflotron CK may be considered as a suitable alternative for decentralized testing sites, especially in situations where creatine kinase results are needed quickly.