Somatic cells in milk were used as a source of DNA and as a substrate for the polymerase chain reaction. Successful DNA extraction required a minimum total of 17 x 10(6) cells in the milk sample. The DNA yield per cell was highly variable but comparable, on average, with the yield from peripheral blood leukocytes. In all cases, direct polymerase chain reaction on milk samples, using a variety of primer pairs, yielded amplified products of correct size that were identical to those obtained by polymerase chain reaction of purified DNA extracted from milk or blood. Milk and DNA extracted from milk served as substrates for direct sequencing of part of the bovine growth hormone gene. Milk samples with added preservative were stored over 200 d without effect on the polymerase chain reaction. Thus, milk can often substitute for blood as a source of DNA for Southern blot analysis and is a preferred substrate for the polymerase chain reaction. The technical convenience of milk as a source of DNA can be expected to increase the field of application of marker-based methods for genetic analysis and genetic improvement of economic traits in dairy cattle.