In the present paper the rat lymphocytes and mouse myeloma cell culture are developed as in vitro test systems and used for the investigation on the biotransformation of drugs. The rat lymphocyte culture was established as a suspension culture after the isolation of the cells from the spleen of male wistar rats by mechanical disaggregation and density gradient centrifugation with a yield of 10(7) cells per spleen and a viability of 80%. The addition of the mitogens PHA and Con A to the culture stimulated the proliferation of the lymphocytes leading to a doubling of the number of cells comparing with control cultures. Myeloma cells are a permanent cell line of B-lymphocytes. The cultivation was carried out as stationary suspension. The marked proliferation of the cells could be increased by addition of Con A. The biochemical properties of both kinds of cells are qualitatively comparable. Cytochrome P-450 mediated demethylase activities could be detected, which were 5-10 fold higher in myeloma cells. The pretreatment with the enzyme inductors phenobarbitone and 3-methylcholanthrene as well as the addition of the mitogens PHA and Con A increased these turnover rates. Reductive and conjugating activities were not present in the cultures. The established and characterized in vitro systems were applied for the investigation on the biotransformation of 4 potential drugs. The cardiac effective Trapidil (Rocornal) derivative AR 12463 (5-piperidino-7-[N-pentyl-N-(beta-hydroxyethyl)]- amino-s-triazolo[1,5-a]pyrimidine) is transformed in lymphocyte and myeloma cell cultures in two compounds. These substances revealed as the hydroxypentyl- and the hydroxypyrimidine derivative.(ABSTRACT TRUNCATED AT 250 WORDS)