A comparison was made of the blastogenic responses of peripheral lymphocytes (BRPL) of normal sheep with those of sheep bearing ovine squamous cell carcinoma (OSCC). In normal tumour-free sheep, BRPL to phytomitogens were found to vary in different age groups, and BRPL to OSCC extracts were found to become significantly elevated in sheep over one year old. In tumour-bearing mature sheep, BRPL to phytomitogens and to OSCC extracts decreased significantly with increasing maturity of tumours. The results are interpreted to show that increasing tumour volume, due to natural growth or enhancement, associates with increasing suppression of cell-mediated responses, and that such suppression may be a cause and a result of increased tumour volume.