Eleven analysts tested contaminated reconstituted (1:10) dry milk powders for penicillin residues using spores of Bacillus stearothermophilus var. calidolacris (Delvotest P method). Three types of responses were noted: positive, negative, and questionable. Prediction equations indicated that 95% of the time, analysts unfamiliar with the technique could detect positive results if penicillin concentrations in samples were 0.010 unit/ml or higher and positive and questionable results if the penicillin concentrations were 0.008 unit/ml. Increasing the reconstitution ratio from 1/11 to 1/4 increased the chances of detecting penicillin in milk powder. Penicillinase added to reconstituted penicillin-contaminated milks in all instances produced negative responses.