The thiol-reactive Alexa Fluor 532 C5 maleimide and its conjugates have excitation and emission spectra intermediate between those of the green-fluorescent Alexa Fluor 488 dye and the orange-fluorescent Alexa Fluor 546 dye, making it an ideal dye for use with 532 excitation sources, including the frequency-doubled Nd-Yag laser.