The effects of low molecular weight plasma inhibitors from rainbow trout (Oncorhynchus mykiss) (RT) were investigated on the carbonic anhydrase enzyme (CA) activities in in vitro human and in in vivo Sprague-Dawley rat erythrocytes. The RT blood was used as extracellular fluid (plasma) source and plasma inhibitors were obtained by dialysis of the plasma. For the in vitro study, human carbonic anhydrase-II (HCA-II) isozyme was obtained by Sepharose 4B-L-tyrosine-sulfanylamide affinity chromatography with an overall purification of about 646-fold. The enzyme (specific activity of 7750 EU/mg protein) was obtained with a yield of 71.1% and SDS-PAGE showed a single band. From in vitro studies, the I-50 value for RT plasma inhibitors obtained was 0.37 mg/ml. From in vivo studies on rat erythrocytes, CA activity was significantly inhibited by the inhibitors from the extracellular fluid of RT for up to 3 h (p < 0.05) following intraperitoneal administration.