The C-terminus of the insulin B chain is essential for dimerisation and expression of negative cooperativity. In order to evaluate the possible physiological role of these phenomena, we have studied the properties in vivo and in vitro of despentapeptide insulin (B26-30 deleted), derived from beef insulin, and deshexapeptide insulin (B25-30 deleted), derived from pork insulin. These materials do not dimerise and have 15% and 0% retention of negative cooperativity respective. Lipogenesis potencies in rat adipocytes were: despentapeptide insulin 19.9 +/- 0.3%; deshexapeptide insulin 19.9 +/- 1.5%. Binding potencies in adipocytes were: despentapeptide insulin 22.6 +/- 7.8%; deshexapeptide insulin 13.2 +/- 3.3%. Metabolic clearance rates were reduced compared to insulin (insulin = 19.1 +/- 0.9; despentapeptide insulin = 9.7 +/- 0.8; deshexapeptide insulin = 6.4 +/- 0.6 ml . min-1 . kg-1 at plasma concentration 0.5 nmol/l). Hypoglycaemic potencies were reduced for both analogues (40% and 30%) when calculated on the basis of plasma concentration although both analogues and insulin were equally effective at lowering plasma glucose concentration in equimolar doses. Plasma half-disappearance time was prolonged (despentapeptide insulin = 7.3 +/- 0.5; deshexapeptide insulin = 9.1 +/- 0.2 min). Both analogues were full agonists and conformed to the general relationship between in vitro and in vivo properties seen with a wide range of modified insulins. They resemble other analogues with modifications which reduce receptor affinity without impairing dimerisation or negative cooperativity. The results do not support a physiological role for dimerisation or negative cooperativity.