Eighteen isolates ofGiardia duodenalisfrom animal and human sources were studied for protein differences by polyacrylamide gel electrophoresis and for antigenic differences by immunoblot analysis. The polyacrylamide gels showed that whilst the isolates were for the most part homogeneous in their protein banding patterns, some isolates did show some differences. The immunoblot analysis yielded many bands, including prominent bands of 32 and 66 kilodaltons. Five of the six isolates that showed differences in protein banding pattern also showed differences in antigenic reactivity. Our findings suggest that differences can be seen with the use of immunoblotting and that this technique is a tool that may be useful for isolate differentiation when used in conjunction with other techniques.Key words:Giardia, giardiasis, characterization, immunoblot.