Electronic somatic cell counts (ESCC) and relative cell volume distributions (RCVD) in milk samples were obtained simultaneously on a Coulter Counter Model TA and their potential assessed, either singly or in combination, for identifying mastitic quarters and cows in 2 herds. Using log (ESCC) alone the probability Pm of misclassifying quarters infected with major pathogens was 6–7 % for herd 1 and 18–1% for herd 2. The inclusion of RCVD variables with log (ESCC) did not substantially improve the chance of detecting quarters infected with major pathogens. However, the RCVD variables did improve the chance of identifying quarters infected by major and minor pathogens compared to the use of log (ESCC) alone, the Pm figures being reduced by about one third.
When testing composite quarter samples, where milk from infected quarters was diluted with that from uninfected quarters, the inclusion of RCVD variables decreased the Pm compared to the use of log (ESCC) alone. In herd 1 the improvement was from 26–1 to 3·0 %, while in herd 2 the effect was less marked, the Pm being reduced from 28–6 to 21–6%. However, as the RCVD assessment is provided simultaneously with ESCC, the additional information is obtained at minimal cost.