It has been shown elsewhere that the epidermal growth factor (EGF) in A431 cells 
can recycle in receptor-bound state (Teslenko et al., 1987; Sorkin et al., 1989, 
1991). Present study deals with the action of primaquine, a lysosomotropic 
agent, on EGF-receptor complexes (EGF-RC). By the method of indirect 
immunofluorescence with anti-EGF-R monoclonal antibody it is found that 
following a 1 h incubation of cells at 37 degrees C in the presence of EGF a 
bright staining of endosomes appears in the intranuclear region, while after 
incubation of the cells at 4 degrees only margins of cells are stained. Such a 
pattern of fluorescence is peculiar of endocytosis in A431 cells. When the cells 
were incubated in the presence of a 0.3 mM primaquine for 1 h, the 
immunostaining is changed: bright compact spot in the para-Golgi region 
appeared. The effect of primaquine is reversible. When the cells after 
preincubation with EGF were incubated in the absence of EGF for 3 h at 37 
degrees C, the staining of cell margins could be observed again, demonstrating 
the recycling of EGF-RC. Under similar conditions of cell incubation, but in the 
presence of primaquine, the staining of the para-Golgi region was not changed. 
In the experiments with 125I-EGF it was shown that intracellular accumulations 
of 125I-EGF were maintained when the cells were incubated in the presence of 0.3 
mM primaquine. It is concluded that primaquine inhibits the recycling of EGF-R 
in A431 cells.