Power density demands for gearboxes are steadily increasing. Higher gear steel qualities can help to meet these demands. Steel manufacturer, as well as customers, usually examine the quality of gear steel batches. For this purpose, ultrasonic immersion testing is a fast and non-destructive evaluation method for the determination of the macroscopic degree of cleanliness. The evaluation follows certain standards and the common influence factors are usually well known. However, the state of knowledge shows that there are still many open questions. Some of these open questions can lead to differences between the results of different laboratories. In the framework of a research project, the macroscopic degree of cleanliness of ultra-clean gear steels had to be determined. However, first results showed significant differences between the results of the laboratories. For this reason, an interlaboratory test was initiated. The same samples were used for the characterizations at the different laboratories. The aim of the investigation was to determine the influencing factors that led to the different results. Based on the interlaboratory test, countermeasures should be defined to ensure a reliable determination of the macroscopic degree of cleanliness of (ultra-clean) gear steels and to ensure comparable results from different laboratories in the future.