A strictly controlled field trial was designed to assess the value of an iodophorpost-milking teat disinfectant in the prevention of new mastitis infections in a 132 cow herd. The disinfectant was applied by a hand spray, a method acceptable, effective, and less costly than conventional teat dipping methods. Only the left side of the udder was treated, the right quarters acting as controls. Dry-cow therapy and other methods of mastitis control were already practised, and no change in routine was introduced apart from half-udder teat spraying. Fifty-eight new cases of mastitis were found to occur on the teat-sprayed side, compared with 91 in the untreated quarters; 35 and 23, respectively, were clinical. No Group B streptococci existed in the herd, and the principal reductions were in Group C streptococci and coagulase-negative staphylococci, the percentages of which were halved. The percentage of coliform infections on the teat-sprayed side was, in contrast, doubled. The incidence of new clinical and subclinical disease without infection was in reverse ratio to that of infections, being 29-9 per cent higher in the quarters which were teat-sprayed. Analysis of cumulative CMT reactions from regular monthly surveys indicated that exponential cell-count values on each side of the udder were identical. Thus, while new infections were significantly reduced, overall cell counts were not, and there is clearly a need for further research into factors other than bacteria which may initiate teat damage and inflammatory changes in the udder.