Effect of interleukin 2 (IL-2) on the proliferative rate of a human colon adenocarcinoma line (HT-29 D4) and of an untransformed rat intestinal cell clone (IEC-6) was studied in vitro. IL-2 had an antiproliferative effect which was cytotoxic-free: interleukin 2 did not induce any significant cell death, apoptosis or change in cellular protein content. Northern blot characterization of cellular mRNA revealed that interleukin 2 enhances the expression of JUN, FOS and TGF beta which are known to be involved in antiproliferative processes. Kinetic analyses suggest that the aforedescribed biological effect is mediated by the AP1 complex (JUN/FOS) when dealing with rapid inhibition (HT-29 D4 and IEC-6) and by an autocrin TGF beta loop when dealing with slow inhibition (IEC-6).