Monthly milk production data collected by DHI supervisors in 28 Virginia dairy herds over a three year period were analyzed. Relationships between somatic cell count and milk production were determined.

A curvilinear relationship between somatic cell count and daily milk yield was shown to exist. The relationship indicated daily milk yield declined more rapidly as somatic cell counts increased from 50 X 10³ through 300 X 10³ cells per ml compared to daily milk decline as somatic cell counts rose from 400 X 10³ through 1,200 X 10³ cells per ml. Daily milk yield declined at a slower rate at somatic cell levels above 400 X 10³ cells per ml.

There was an apparent cumulative or continual relationship between somatic cell count and daily milk yield. Losses and declines in test day milk production were greater when cumulative somatic cell count was considered.

Complete lactation records of 305-day, mature equivalent milk production were compared to weighted, lactational somatic cell counts. Lactational milk production losses were not as great as daily milk production losses, extended to full lactation, indicated. Design difficulties and inadequacies of models used in complete lactation evaluation may explain the discrepancies.

A maximum somatic cell count of 150 X 10³ to 200 X 10³ cells per ml was indicated as an optimum level for somatic cell count. Above this level milk losses were excessive.