The relationship between Dairy Herd Improvement program test-day milk yield and somatic cell counts in milk was analyzed in 34 dairy herds over 3 yr. Estimates of this relationship were more accurate when somatic cell counts were transformed to natural logarithms rather than actual. For 67,707 observations, test-day milk yield decreased with increasing somatic cell count. The decrease of milk yield for second and later lactations, as somatic cell count increased, was greater than for first lactations. When herds were partitioned according to herd milk production (greater than 7700, 6500 to 7700, and less than 6500 kg/yr), decrease of milk yield was linear with increasing somatic cell counts for herds averaging below 7700 kg milk. Regressions were linear, quadratic, and cubic for both parity groups in herds with high production, over 7700 kg/yr. Quarter samples were composited for each cow and cultured. Percentage of infected cows increased as somatic cell counts increased with greater infection rates above 400 X 10(3).