We are unaware of quantitative data on amino acid (AA) levels in CF plasma, leukocytes, and cultured cells, although alterations in methionine metabolism have been suggested in CF. Plasma samples and washed leukocyte pellets obtained after overnight fasting from clinically stable CF patients, obligate heterozygotes, and normal (NL) subjects, ages 16-30, were processed for AA analysis by standard methods. CF and NL fibroblasts matched for age and passage # were cultured in 3 different media (alpha, Eagle's MEM, and MEM + non-essential AA), harvested at confluency, and intracellular AA measured. Methionine content of all CF samples was similar to NL. Compared to NL (n=6), CF plasmas (11) had an ↑ mean level of glycine (CF 298 nmol/ml ± 12.4 [SEM], NL 231 ± 15.1, p <.005) and marginally altered citrulline and phenylalanine; for these AA, heterozygote means fell between CF and NL. CF and NL ranges for all plasma AA overlapped. CF leukocytes (5) compared to NL (3) had reduced leucine (CF 2.54 ± .46, NL 5.00 ± .29), isoleucine (CF 1.01 ± .30, NL 2.51 ± .12), valine (CF 1.52 ± .13, NL 2.94 ± .39), and phenylalanine (CF 0.86 ± .17, NL 1.95 ± .20) (all p <0.02) expressed as % of total AA; CF and NL ranges did not overlap and heterozygotes had decreases like CF for these four AA. The CF and NL fibroblasts (2 lines each) had similar AA content in all 3 media. The alterations noted in CF AA are relatively small, and have no obvious relationship to the major clinical or biochemical features of CF.