Growth rate, sugar transport, and amino acid transport of yeast cells grown at 12 degrees C were inhibited by cool-white fluorescent light. At light intensities below 1,250 lx, growth and membrane transport were only slightly inhibited. Above 1,250 lx, there was increasing inhibition of both processes. Transport of histidine was completely inhibited after 3 to 5 days in cultures grown at 12 degrees C under 3,500-lx illumination. Cells grown at 20 degrees C were not inhibited by light intensities that caused complete loss of viability and membrane transport activity in cells grown at 12 degrees C.