We established it positive effect of the nonionic surfactant Triton X-100 on the permeability of Yarrowia lipolytica cells to p-nitrophenyl phosphate, a substrate of acid and alkaline phosphatases. Maximal permeabilization of the cells was achieved with 0.1-0.2% of Triton X-100. The enhanced cell permeability depended to a greater extent on the detergent concentration than on the ratio of cells to the detergent. The lack of proteolytic activity released from the vacuoles, the relatively low amount of phosphatases present in the supernatant after treatment with the detergent, as well as electron microscopy observations showed that the permeabilization of yeast cells with Triton X-100 is a mild process. The procedure is simple and rapid, and may be used for the assay of enzyme activities associated with surface structures.