A study is reported of the effect of preincubation of raw and laboratory pasteurized bulk milk samples on the tyrosine value (TV) of milk and its relationship with bacterial populations. For raw samples, mean TVs before and after preincubation for 20±2 h at both 25 and 30 °C differed (P < 0·001). After preincubation at 30 °C, the TV of 47 % of raw samples exceeded that considered to represent the upper limit of variation in the TV of a normal bulk milk supply. At this temperature, however, there was no indication that proteolytic psychrotrophs were a major contributor to increased TV. With preincubation of raw samples at 25 °C, an increase to above the upper limit of variation occurred in only 20 % of samples; however, proteolytic psychrotrophs appeared more active at this temperature. In similar samples preincubated after laboratory pasteurization, large increases occurred in TV after 20 ±2 h at 30 °C, while at 25 °C the increases were much smaller and of the order found for preincubated raw samples. The results showed, however, that nonpsychrotrophic bacteria were also the principal source of proteinase giving rise to increased TVs in laboratory pasteurized samples.