The survival of two microbial contaminants, Enterobacter cloacae and Pseudomonas paucimobilis, in yoghurts manufactured from cow's milk and soymilk was investigated during storage for 45 days at 4 and 12 degrees C. Sensory panel tests performed before microbiological investigation, showed that the flavor of soy-yoghurts made with cocoa powder or malt added did not have the beany taste of soy beans. Both contaminants were significantly resistant to low pH values during storage for 32 days at 4 degrees C. The survival at 4 degrees C was remarkable in both plain and flavored yoghurts and a population close to 10(2) C.F.U./ml was observed after 38 days of storage. Experiments performed with soymilk yoghurts showed an enhanced survival of P. paucimobilis at 4 degrees C compared to the storage in cow's milk yoghurts; microbial values were close to 7-8 x 10(6) C.F.U./ml after 16 days. Soymilk exhibited a protective effect on L. delbrueckii subsp. bulgaricus and S. thermophilus at 12 degrees C and, compared to the survival in cow's milk yoghurts, a larger number of viable cells of both probiotic microorganisms (10(6) and 10(8) C.F.U./ml, respectively) were observed after 36 days of storage.