Sixty-two strains of yeasts, molds and bacteria were screened for lactase (beta-D-galactosidase) activity. Strains exhibiting the enzyme activity were evaluated for cell yield as well as enzyme units available per litre of the medium, per g cell dry weight and per mg protein of their cell-free extracts. The molds exhibited lowest enzyme activity but highest cell yields, bacteria produced lowest cell yield and maximum enzyme activity. Cultures exhibiting very high activity among yeasts were Saccharomyces fragilis (strain 3217) and among bacteria Streptococcus cremoris (strain H), Lactobacillus bulgaricus (strain RTS and 1373) and Leuconostoc citrovorum (strain 8081).