In contrast to adherent cells, human B and T lymphoblasts, marmoset monkey T lymphoblasts, and mouse T lymphoblasts do not form monolayers and have a poor ability to pinocytose. After a 10-min incubation of lymphoblasts at 37 degrees C, the level of internalized medium reached a plateau. During this time, lymphoblasts pinocytosed 3-4 femtoliters (1 fl = 10(-15) l) of medium per cell as calculated by the quantity of the entrapped pinocytic marker 5(6)-carboxyfluorescein. The levels of pinocytosed liquid did not increase during a subsequent 90-min incubation of cells at 37 degrees C. Adherent HeLa cells took up 27 fl of medium per cell per hour. Other types of adherent cells were reported by others to pinocytose 20 to 90 fl of medium per cell per hour. The process of pinocytosis in lymphoblasts appeared to be reversible since cells which were pre-loaded with carboxyfluorescein and then incubated at 37 degrees C in fresh medium lost the marker almost completely within 40 min. Similar results were obtained with horseradish peroxidase as the pinocytic marker. Further evidence that lymphoblasts have a low capacity for pinocytic internalization relative to adherent cells was obtained from the observation that Namalwa lymphoblasts were approximately 100 times more resistant to the cytotoxic action of the protein toxin gelonin than the adherent HeLa cells. Gelonin is a ribosome-inactivating toxin which is not capable of binding to cells, and its only mode for internalization appears to be pinocytosis. Ribosomes in cell lysates of the two lines were equally sensitive to gelonin. It is speculated that the poor pinocytic ability of lymphoid cells may reflect a fundamental difference between adherent and non-adherent cells and that this may impede the targeting of drugs into lymphoid cells.