Résumé : A covalent conjugate of alpha‐foetoprotein (AFP) and horseradish peroxidase (HRP) has been used to follow, at the ultrastructural level, the pathway of AFP uptake and translocation in a rat rhabdomyosarcoma cell line. The cells were incubated for several times at 4°C and/or 37°C, and fixed. AFP‐HRP was found to enter the cells via coated pits and receptosomes and to move to tubular elements of the trans‐reticular portion of the Golgi. Some observations suggest that AFP can be recycled back to the cell surface. On the other hand, the cells were incubated with a noncovalent conjugate of AFP and 3H‐arachidonic acid [3H‐(20:4)], and the uptake of the fatty acid molecules studied by ultrastructural autoradiography. The cytoplasmic labeling, very low after an incubation in the presence of [3H‐(20:4)]‐AFP for 2 hours at 4°C, increased rapidly after transfer of the cells for 5 minutes to 37°C. These observations support the hypothesis that AFP plays a role in the intracellular delivery of polyunsaturated fatty acids. Copyright © 1986 Wiley‐Liss, Inc.