This study was conducted to determine the duration of lactation, lactation and 305 days milk yield, milk fat and protein percentage and somatic cell counts of Holstein-Friesian cattle selected randomly in five private farms in England and to investigate the effects of some environmental factors on these traits. Material of the study was 2514 yield records collected from Holstein-Friesian cattle between the years 1994 and 2003. In the statistical analysis of the yields, contrast-test of the GLM procedure in SAS programme package was used. General average of the duration of lactation, lactation and 305 days milk yield, milk fat and protein percentage and milk somatic cell counts were 324.32 days, 7715.23 kg, 7218.62 kg, 4.028%, 3.333% and 137,948 cell/ml respectively. During the study, the farm where the animals were kept and the year in which lactation started made significant effects at P<0.001 level on all traits, also the turn of lactation has made a P<0.05 level effect on these traits except the P<0.05 level effect on the duration of lactation. Season had no important effect on lactation milk yield and somatic cell count in milk. However, season, like other factors made significant effects on the duration of lactation, milk fat and protein rates at a level of P<0.001 and on 305 days milk yield at a level P<0.01. The results of this study showed that the farms achieved high levels of milk yield by providing optimal conditions. Management conditions and the use of high yielding breeders accomplished the improvement in yields in different years. Two principle factors came forward in the study and highest determining factors were observed to be the farm and turn of lactation.