The callus of Lemna minor L. (duckweed) is capable of heterotrophic assimilation of galactose and beta-lactose, whereas the intact plant is not. Sucrose-grown callus adapted rapidly, uniformly, and reversibly to utilization of galactose or lactose in heterotrophic growth. Single-nutrient deficiencies of callus growth medium were corrected fully by milk whey; the lactose of milk whey was utilized Lemna callus grows well at pH 4.0-6.5 and tolerates 0.05% (5.5 mM) lactic acid at pH 5.0 with slowed growth. Preliminary data using elude extracts of callus revealed higher specific activities of enzymes of lactose and galactose metabolism : beta-galactosidase, galactokinase, UDP gal pyrophosphorylase, and inorganic pyrophosphorylase. Lemna minor callus should be able to consume sweet-cheese whey.