Poly(A)+ RNA from C6-BU-1 rat glioma cells and rat astroglial cells induced isoleucine transport activity when injected into Xenopus laevis oocytes. The Na+-independent component of isoleucine transport was inhibited by leucine, phenylalanine and 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) but neither by methylaminoisobutyric acid (MeAIB) nor lysine. A Km value of approx. 100 microM was determined for the Na+-independent transport of isoleucine. These data are in accordance with expression of a system L like transporter. By injection of size fractionated poly(A)+ RNA a length of approx. 1.9 kb was determined for the pertinent mRNA.