In this study indirect immunofluorescence was performed on both live and fixed trophozoites to determine the level of variability in surface antigen expression between 14 Giardia duodenalis isolates, using a monoclonal antibody raised against the Portland 1 isolate (ATCC No. 30888). Subsets of antigen positive cells were detected in 13 isolates ranging in number from < 1% to 50% of the total population. The differences in antigen expression between 10 uncloned isolates did not correlate with genetic differences determined using isoenzyme analysis. Examination of four clones of the Portland 1 isolate showed that all of the progeny expressed surface antigen at significantly different levels to the parent.