The substrate selectivity of the neutral amino acid transport systems of the b30 clone of the choriocarcinoma cell line (BeWo) were characterized. Three transport systems were identified in undifferentiated (without forskolin) and two transport systems in differentiated syncytial cells (with forskolin). In the undifferentiated b30 cells were two sodium-dependent systems with one having substrate selectivity patterns resembling system A (e.g. sensitive to MeAIB and a broad range of neutral amino acids) and the other resembling system ASC (e.g. MeAIB insensitive and inhibited by alanine, serine and cysteine). In addition a sodium-independent system was identified with characteristics resembling system l. The differentiated syncytial cells possessed the system A and system l-like activities but not the system ASC-like activity of the system A-like or system l-like activities. The b30 clone is apparently an appropriate model for placental neutral amino acid transport systems.