A technology for magnetite nanoparticles surface modification by polyethylenimine was proposed. The sizes of the nanoparticles using transmittance microscopy method were determined. The influence of pH, time of mixing, sorbent and antibiotic concentration on the enrofloxacin sorption was investigated and optimal conditions of extraction were found. Enrofloxacin sorption from plasma-modelling solution was studied and was shown that that in optimal conditions the extent of sorption is 78% and desorption from magnetite modified nanoparticles using ethanol-sodium dodecylsulfate is 95%.