After serial transplantation in nude mice, we had established three new human choriocarcinoma cell lines (NaUCC-1,2 and 3). These three cell lines and BeWo were examined for sensitivity to Act-D, MTX and the combined agents (Act-D + MTX) 3H-Act-D uptake and 3H-MTX uptake, and were compared for each treatment. NaUCC-1 showed low sensitivity to MTX (p less than 0.05), but showed high sensitivity to Act-D (p less than 0.05). BeWo showed low sensitivity to Act-D (p less than 0.05). In examining sensitivity to the combined agents (Act-D+ MTX), the sensitivity of NaUCC-2 to Act-D was decreased (p less than 0.05) by MTX added at the same time. In the 3H-Act-D uptake experiment, NaUCC-1 did uptake a relatively larger amount of 3H-Act-D (p less than 0.01) and BeWo a smaller amount of it (p less than 0.05). In the 3H-MTX uptake experiment, NaUCC-1 did uptake a relatively smaller amount of 3H-MTX (p less than 0.05), but NaUCC-2 uptook a larger amount of it (p less than 0.005). NaUCC-1 established from the patient in whom tumor cells were resistant to treatment, had a low response to MTX. NaUCC-2 was established from the patient in whom it was found that MTX inhibits the Act-D effect on tumor cells. In the study of combined agents in NaUCC-2, the growth inhibition effect of Act-D was suppressed by the MTX added.