Wireless Sensor Networks (WSNs) are an emerging technology that will be widely deployed in the near future. Most WSNs operate on the 2.4 GHz band of the three free ISM frequency bands. The 2.4 GHz frequency band is already used by different wireless systems. With an increasing number of WSNs, the scenario of different WSNs operating on the same IEEE 802.15.4 frequency channel becomes more likely. WSN Medium Access Control (MAC) Protocols used today were not designed with this problem in mind. To date, the research focused on interference on the Physical Layer. This work analyses the jamming potential and the robustness of MAC Protocols, namely X-MAC and Low Power Probing (LPP), at the level of inter-network competition for medium access, when multiple WSNs are in range of each other operating on the same channel. The following parameters have been investigated and their effect on interference is shown: sampling time, channel check rate and payload.