Web Services technology provided several advantages over other technologies, however it still has serious limitations, including high latency and high protocol overhead. To improve performance, SOAP network traffic needs to be substantially reduced. This paper presents a novel approach, called similarity-based SOAP multicast protocol (SMP), to address the issue of latency. SMP reduces network traffic by aggregating syntactically similar SOAP messages to form a "compact SMP message";. The addresses of clients are encoded as strings in the SMP message header. The similarity of SOAP messages is measured in pairs and is based both on the message template and on the values of each XML tag in the messages. Each XML node in a SOAP message is indexed with an identifier and its position in the SOAP message. Only the indexed form of a SOAP message is sent to clients. Intermediary routers along the paths from server to clients parse the content of each SMP message passing through them and perform necessary operations to forward it to neighbouring routers. Experiments show that SMP can achieve up to 70% reduction in network traffic compared to traditional SOAP unicast.