This study was carried out to investigate the effect of somatic cell count (SCC) on milk yield and milk composition in first and second lactation Holstein dairy cows. Thirty cows in first lactation and 49 cows in second lactation were used in the study. Animals were 15 +/- 9.87 days in milk. Individual milk samples were collected monthly from June 2009 to March 2010, and somatic cell counts, milk protein, milk fat, lactose and milk urea-N were determined. Four SCC groups were formed for determining effect of SCC on milk yield and composition. These groups were as follows: <= 200, 201-500, 501-999, and >= 1.000x10(3) cell/mL. It was observed that SCC had a high significant effect on milk yield, milk protein, milk lactose (P<0.01), total solids and milk urea-N (P<0.05), however, the effect of SCC on milk fat was not significant (P>0.05). This study indicates that high SCC negatively affects not only milk yield but also milk composition and quality.