The value of the classical blood variables (erythrocyte count, hemoglobin, hematocrit) and their derivatives (mean erythrocytic volume, mean erythrocytic hemoglobin, mean erythrocytic hemoglobin content) as indicators of fish health is assessed from the viewpoints of measurement precision and the distortion of information content by sampling procedures and storage conditions. So also is the concept of hematological norms and their validity in relation to natural fish populations. Few of the assumptions underlying the use of hematological data for health assessment purposes can be regarded as other than marginally valid. Of the primary indices, erythrocyte numbers and hemoglobin, if corrected for nonviable cells and accompanied by information on isomorph abundances, appear to be the more useful indicators of O2carrying capacity. Hematocrit and hematocrit-dependent Wintrobe indices emerge as suspect. Higher-order indices describing red cell anisocytosis and yielding an “erythron profile” provide additional and more sensitive means for assessing blood status.