In a two-year study of the incidence of subclinical mastitis in two beef suckler herds over 2400 quarter milk samples from 180 cows were examined. Somatic cell counts and total bacterial counts were carried out and infecting organisms were isolated on sheep blood agar. Results of these tests indicated that: (a) in spite of extremely dirty udders, fewer contaminants were found in the suckler cow milk samples than in a group of over 1700 samples from typical dairy herds; (b) 18 per cent of all quarter milk samples were infected; (c) 67 per cent of all infections were due to staphylococci and 20 per cent to streptococci; (d) 56 per cent of staphylococcal infections were associated with cell counts less than 500,000 per ml compared with 39 per cent of other infections; (e) 70 per cent of samples had somatic cell counts less than 500,000 per ml. (f) 20 per cent of samples had somatic cell counts over 1 million per ml; (g) only 36 per cent of samples with cell counts over 1 million per ml were associated with udder infections; (h) high cell counts and udder infections were more frequent in early than in mid-lactation. Due to a shortage of animals in the late lactation little evidence was available to support the contention that cell counts rise and the numbers of infected quarters increase towards the end of lactation.