Mycobacterium paratuberculosis is the agent of a chronic, fatal, granulomatous enterocolitis in ruminants. Most susceptible to infection are animals in their first month of life, although clinical disease becomes apparent only several years later. The main way of infection is the fecal-oral route, but since excretion of M. paratuberculosis has been demonstrated in colostrum as well as in milk of infected cows, concern aroused about feeding mycobacteria loaded colostrum to highly susceptible neonate calves. We investigated the effects of colostrum pasteurization under laboratory conditions, i.e., elimination of the mycobacteria and preservation of immunoglobulins through heat treatment: colostrum samples were spiked with M. paratuberculosis (ATCC 19698) in concentrations of 10⁴, 10³ and 10² CFU/ml each, and split into two groups, pasteurized and unpasteurized. The pasteurized samples were held for 30 minutes -exclusive of come-up time- at 63°C in a water bath, then cooled on ice. Both groups of samples were subsequently handled similarly: IgG content was measured by single radial immunodiffusion on agarose gel, and the samples were cultured on Herrold's Egg Yolk Medium supplemented with mycobactin J for 16 weeks at 37°C. The mean value for colostral IgG was 44.4 g/l in the unpasteurized samples and 372 g/l in the pasteurized samples, with a mean decrease in IgG concentration of 12.3%, a statistically significant difference (p=0.005). High quality colostrum (>48 g/l IgG) showed a significantly larger mean percental IgG loss than colostrum of lesser quality (20.1% vs 6.7%; p=0.0001), but did not completely eliminate M. paratuberculosis from the pasteurized colostrum samples. Although not sufficient by itself to prevent potential infection of neonate calves by feeding colostrum from infected and shedding cows, colostrum pasteurization may be advisable, along with other control measures, to reduce the infection pressure in infected herds.