The pH, pC0, and PO, values for diseased fish did not differ significantly from those values for healthy fish, but fish treated with terramycin showed a highly significant rise in pC0, with a corresponding fall in PO,. Haematocrit values, as well as haemoglobin and total plasma protein concentrations were significantly lower in diseased fish while treated fish showed a significant rise in total plasma protein. Glucose concentrations fell very significantly in diseased fish and showed a significant increase in treated fish.The plasma electrolytes, sodium, potassium and chloride, were significantly reduced in diseased fish and potassium levels also fell in treated fish. Diseased fish showed very significantly reduced erythrocyte counts. Of the three parameters MCV, MCH and MCHC, only the values for MCH in diseased fish differed significantly from those for healthy fish.

Osmotic fragilities and erythrocyte sedimentation rates (ESR) were measured in healthy, diseased, treated and recovered trout, Salmo gairdneri. Bacterial infection with Aeromonas and Streptococcus resulted in more fragile erythrocytes and raised ESR values. Treatment with antibacterial agents improved the values of both parameters. Possible reasons for such changes are discussed. Caution in the use of apparently healthy fish for the unqualified interpretation of haematological results is expressed.

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