The relationship between the mitogenic lectin PNA and other mitogenic and non-mitogenic lectins was investigated. PNA labelled with 125I was found to bind equally well to T and B lymphocytes, after neuraminidase treatment, with 3.88 times 10(6) and 3.73 times 10(6) binding sites respectively. Only the T cell fraction was stimulated, however, and only after neuraminidase treatment. Preincubation of neuraminidase-treated cells with non-mitogenic lectins and antiserum which appeared to bind to the same receptor as PNA, enhanced the latter's stimulatory properties. Capping and co-capping techniques were used to examine the topology of lectin receptors on the lymphocyte surface. The receptor glycoprotein for the mitogenic PNA lectin was found to be distinct from that possessing the Con A and PHA receptors, as well as that possessing the receptor for the non-mitogenic lectin from Helix pomatia.