1. Measurements of T-lymphocyte surface ferritin using flow cytometry show that phytohaemagglutinin (PHA) stimulation causes a marked increase in the number of cells bearing spleen-type (S) and heart-type (H) ferritin on their membrane, whereas no such change occurs in non-stimulated cells. This coincides with increases in interleukin-2 receptors, transferrin receptors and HLA-DR antigen.
2. There is an increase in the intracellular concentration of both S- and H-ferritin in lymphocytes after PHA stimulation: H-ferritin increases five- to seven-fold, but S-ferritin only two- to three-fold. The maximum H/S ratio is about 15/1. However, these increases also occur in cells cultured in the absence of PHA.
3. Small amounts of both S- and H-ferritin are released into the medium, especially from stimulated cells, but the H/S ratios are lower than intracellular ratios.
4. The present findings suggest that lymphocyte stimulation followed by ferritin synthesis is accompanied by an increase in the amount of intracellular and cell surface ferritin and, possibly, the amount released from the cells.