The diagnostic accuracy of conventional methods for the diagnosis of subclinical mastitis, such as the direct microscopic count (DMC) and electronic cell count (ECC) either used alone or in combination with bacteriological examinations, according to international standards, were compared with DNA determinations and a radial immunodiffusion test (MMT). The latter is based on single radial immunodiffusion of bovine serum albumin (BSA) in agarose containing anti-BSA serum. This investigation was conducted on 1 008 foremilk samples, collected via the teat canal from 179 dairy cows, plus 56 samples obtained from 14 of them by means of cisternal puncture. It is concluded that BSA concentrations in milk causing an MMT diameter ≥ 8mm are diagnostic for mastitis in lactating cows under the conditions existing in this experiment. The coefficient of variation of the MMT (14, 67%) is significantly smaller than that of the DMC (87, 78%), ECC (72,25%) and DNA content (37, 19%) respectively and lies within the 15% limit recommended for diagnostic methods. Diagnoses made by MMT, DMC and ECC over 3 successive days varied by 4,1%; 14,5% or 28,4% respectively. The reduced repeatability of the MMT resulted from genuine changes in udder health whereas 10,1% and 24,3% of variance observed for the DMC and ECC respectively were due to other factors. In comparison to the MMT, mastitis diagnosis based on international standards resulted in 43,13 ± 20, 8% false positives. These are mainly due to teat canal infections simulating mastitis. When both the MMT and international mastitis standards were used it was possible to distinguish between quarters with irrelevant and relevant teat canal infections, non-specific cellular reactions and septic or aseptic mastitis without having to resort to cisternal puncture. Staphylococcal beta toxin inoculated into the teat canal, facilitated studies on the sequence of events leading to elevated BSA levels and cellular counts in the udder. A pre-inflammatory leucocytosis, resulting from passage of small amounts of toxin into the teat cistern was shown to occur in this investigation. The diagnosis of subclinical mastitis in dry cows and within the first week post parium is as inaccurate by means of the MMT as by conventional methods.