Selenoprotein P was purified from human plasma using conventional chromatographic methods featuring metal-chelate-affinity chromatography as the final step. Two distinct isoforms with different selenium content were isolated and identified by N-terminal sequencing and immunoblot analysis. Their molecular mass is 61 and 51 kDa, respectively. Both isoforms could be detected in fresh plasma from five individuals. This rules out the possibility of the second isoform being an artifact which results from degradation of full-length selenoprotein P during purification.