Berberine, a natural cationic colorant was successfully employed onto nylon 66 fiber in this research. The effects of three important variables, namely pH, temperature, and liquor ratio were examined on the % exhaustion, color strengths, and color yields of the sample. It has been employed in antimicrobial finishing as a natural agent on nylon 66 due to its characteristics of cationic quaternary ammonium salt. Antimicrobial activity of the sample was studied against Staphylococcus aurens (ATCC 6538) and Klebsiella pneumoniae (ATCC 4352) according to test method KS K 0693-2001 and the corresponding berberine finished sample showed very effective antimicrobial functions showing about 99.9% of bacterial reduction against above-mentioned two bacteria. The maximum % exhaustion, color strengths, and color yields were obtained at 98 degrees C, alkaline condition (pH 11) and lower liquor ratio (20 : 1). An appropriate predictable empirical models were also developed using Excel solver function incorporating interaction effects of all variables to predict the % exhaustion, color strength(K/S), and the satisfactory results (R-2 > 0.99) were obtained. (c) 2006 Wiley Periodicals, Inc.