A new direct colorimetric assay of microcystin in water and algal samples is proposed consisting of two procedures as follows: 1) the elimination of phosphorus in the sample and concentration of microcystin using a C(sub)18 cartridge, 2) the detection of the released phosphorus by the ascorbic acid method and determination of protein phosphatase (PP) inhibition by microcystin. The optimum amounts of phosphorylase ${alpha}$ and PP-1 in 50 ${mu}$L concentrated sample were 50$mu extrm{g}$/50${mu}$L buffer and 1.0unit/50${mu}$L buffer, respectively, for the best assay. The pH for the maximum activity of PP-1 was 8. The minimum detectable concentration for this method was about 0.02$mu extrm{g}$/L, which is sufficient to meet the proposed guideline level of 1$mu extrm{g}$ microcystin/L in drinking water. Consequently, it would seem that the proposed direct colorimetric assay using PP is a rapid, easy, and convenient method for the detection of microcystin in water and algal samples.