Adherent GFAP-positive cells of neocortical origin in vitro produce and release members of three families of sulphated proteoglycans and a sulphated protein that copurifies with heparan sulphate proteoglycan (HSPG). Conditioned medium (CM) and the proteoglycans contained in the CM have neurite growth-promoting activity when immobilized on defined substrates of growth but not when in the nonimmobilized compartment. On a poly-D-lysine substrate, the rank ordering of specific neurite growth activity based on protein concentration was 330 kDa HSPG >> 100 kDa HSPG/chondroitin sulphate (CS) PG mixture or hybrid > 330 kDa CSPG > 50 kDa CSPG/dermatan sulphate (DS) PG mixture or hybrid and the 31 kDa sulphoprotein. Astrocyte CM lost its growth facilitatory activity when prepared and released by astrocytes in the presence of soluble mediators of inflammation. Loss of activity could not be explained by qualitative or quantitative alterations of released proteoglycans but appeared to be associated with the presence of an inhibitor. The sulphoprotein that copurified with HSPG was a potent inhibitor of HSPG-mediated neurite growth.