Transport of L-lysine by a cultured placental trophoblast cell line was investigated by characterization of L-[3H]lysine uptake. In the mononuclear form of the BeWo clone b30 choriocarcinoma cell, at least two sodium-independent systems are present. Concentration dependence data were fitted by a two system model with K(m) values (± s.e.) of 2 ± 0.7 and 94 ± 31 μM and V(max) values (± s.e.) of 0.7 ± 0.3 and 25 ± 6.0 nM/mg DNA/min. A portion of sodium-independent uptake was inhibited by the sulphydryl modifying reagent N-ethylmaleimide (NEM). Following NEM treatment, the data were fitted by a single system with K(m) = 10 ± 2 μM and V(max) = 5.1 ± 0.8 nM/mg DNA/min. In the absence of sodium, NEM-resistant uptake was sensitively inhibited by leucine whereas NEM-sensitive uptake was not inhibited by leucine. It is concluded that like placental vasal membrane, the mononuclear BeWo cell possesses two sodium-independent L-lysine transport systems. The high-capacity, NEM-sensitive leucine-insensitive system resembles the widespread system y+. The high-affinity, NEM-resistant, leucine-sensitive system resembles system b0,+.