Eucaryotic expression vectors bearing a 1.4 kb cDNA encoding the 4 repeat isoform of human tau, tau441, in either the sense or anti-sense orientation with respect to a cytomegalovirus (CMV) promoter were constructed. The resulting constructs were used to transiently express tau in Chinese Hamster Ovary cells and to generate non-neuronal stable cell lines. Immunocytochemical studies of these cells show that tau is expressed in the sense but not the anti-sense or vector containing lines. Some of the cells expressing tau showed fine elongated processes which were stained by tau antibodies. The general tau immunostaining pattern appeared diffuse and punctuate. The expressed tau was seen both unbound and bound to microtubules. In some cells labeling with antibodies that specifically recognize hyperphosphorylation of tau was observed. The size of this population increased with increasing numbers of cell passages. However, no increase in steady-state tubulin level was observed following tau441 expression. These studies show that tau can accumulate in the cells without a concomitant increase in tubulin.