The mechanisms of iron accumulation by cultured hepatocytes isolated from fetal rat liver (19 days gestation) were investigated using rat transferrin labeled with 125I and 59Fe. The rates of iron and transferrin internalization by the cells were measured by incubating the hepatocytes with the labeled transferrin at 37 degrees C followed by treatment with pronase at 4 degrees C to remove surface-bound transferrin and iron. Iron internalization increased linearly with time. Approximately 65% of the internalized iron was incorporated into ferritin. In contrast to iron, the rate of transferrin internalization was biphasic, with a rapid phase during the first 10 to 15 min and a second slower phase which becomes more apparent after that time. Iron and transferrin internalization were temperature-dependent. Chase experiments showed that the internalized transferrin donated all of its iron to the cell and was then released in a biphasic manner which was dependent on the time of preincubation with radiolabeled transferrin. These experiments showed that iron uptake occurs by at least three processes. The first mechanism involves the specific receptor-mediated endocytosis of transferrin. Each cell has an average of 7.8 +/- 1.0 X 10(5) (mean +/- SE, n = 5) transferrin binding sites with an apparent association constant of 2.0 +/- 0.4 X 10(6) M-1. The second process is nonsaturable up to a transferrin concentration of at least 6 microM but like the specific process, also leads to accumulation of iron in excess of transferrin. It involves the endocytosis of transferrin mediated by 4.2 X 2.6 X 10(5) M-1.(ABSTRACT TRUNCATED AT 250 WORDS)