The antiviral antibiotic brefeldin A (BFA) strongly inhibits the protein secretion in cultured rat hepatocytes (Misumi, Y., Misumi, Y., Miki, K., Takatsuki, A., Tamura, G., and Ikehara, Y. (1986) J. Biol. Chem. 261, 11398-11403). We have further examined the inhibitory effect of the drug on intracellular transport of albumin by an immunocytochemical technique with peroxidase-conjugated Fab fragments of anti-rat albumin IgG. In hepatocytes treated with BFA (2.5 micrograms/ml) for 1 h at 37 degrees C, no characteristic structures of the Golgi complex could be observed, and albumin was diffusely distributed in the endoplasmic reticulum (ER), nuclear envelope, and small vesicles around, in contrast to its condensed localization in the Golgi complex in the control cells. Such an unusual distribution of the secretory protein, however, was rearranged to the normal localization in the Golgi complex after 4 h even in the presence of the drug, possibly due to a metabolism of the drug to an inert form. Exposure of the cells to BFA with constant renewals (2.5 micrograms/ml at 1-h intervals) or at a higher concentration (10 micrograms/ml) caused a prolonged accumulation of albumin in the ER, resulting in its dilation. These results indicate that BFA primarily blocks the protein transport from the ER to the Golgi complex, consistent with the biochemical data previously reported.