Methylisothiazolinone (MI) is an isothiazolinone-derived biocide used for controlling microbial growth in industrial and household products, often in a mixture with 5-chloro-2-methyl-3-isothiazolone (MCI). MI is active against Gram-positive and Gram-negative bacteria, fungi, and yeast with MIC values of 0.0045, 0.0015, >0.01, and 0.0065% (w/w) for S. aureus, P. aeruginosa, A. niger, and C. albicans, respectively, when used alone. MIC values are 7 to 200-fold lower when MI is used in combination with MCI. MI decreases neurite outgrowth of rat cortical neurons when used at concentrations of 0.1-3 µM and inhibits Src family kinases in cell-free assays. MI, alone and as a mixture with MCI, can elicit contact sensitization.