Galectin‐3, a mammalian galactoside‐binding protein, is not expressed in the Jurkat T‐lymphoblastoid cell line. However, Jurkat cells express surface glycoprotein receptors for galectin‐3, one of which is shown to be the glycosylated heavy chain of CD98 (4F2 antigen), a T‐cell activation marker. Addition of galectin‐3 to Jurkat cells triggers a sustained influx of extracellular Ca2+ in a concentration dependent manner. The induced increase in cytosolic [Ca2+] i is blocked by sugar hapten inhibitors of galectin‐3. The galectin‐3‐induced effect is insensitive to voltage‐gated Ca2+ channel antagonists such as prenylamine, nifedipine and diltiazem and to pertussis toxin but is inhibited by cholera toxin. The results suggest that galectin‐3 released by accessory cells such as macrophages may bind in vivo to T‐cell activation antigens and also participate in Ca2+ signalling.