Plasma melphalan levels have been measured in nine (mostly stage IIIA) multiple myeloma patients after therapeutic doses of drug had been given p.o. and i.v. A new isocratic high-pressure liquid chromatographic (HPLC) method with a sensitivity limit o 5 ng/ml was used to quantify the melphalan. Patients receiving 8-28.5 mg melphalan i.v. showed alpha and beta plasma decays with half-lives of 7.7 +/- 3.3 (mean +/- S.D.) and 83 +/- 14 min respectively. The apparent volume of the central compartment was 12.8 +/- 4.3 1, and the total volume of distribution was 0.62 +/- 0.21 l/kg. Very variable absorption was seen in the same patients after receiving 5-12 mg melphalan p.o. The half-life of the absorption phase varied from 2.1 to 62.1 min (22.8 +/- 18.1 min) with delays (before absorption started) of 0-113 min. The fraction of dose absorbed varied from 0.32 to 1.03 (0.72 +/- 0.23), and the half-life of the beta phase was 92 +/- 27 min. The type of breakfast eaten before p.o. melphalan was found to correlate with the fraction of drug absorbed.