3-(2-Furoyl)quinoline-2-carboxaldehyde (FQ), Chromeo P465, and Chromeo P503 are weakly fluorescent reagents that react with primary amines to produce fluorescent products. We studied the reaction of these reagents with alpha-lactalbumin by mass spectrometry. The reaction generated a set of products by the addition of one or more labels to the protein. At room temperature, the reaction was an order of magnitude faster with the Chromeo reagents than with FQ; however, the steady-state labeling efficiency was a factor of two higher for FQ compared with the Chromeo reagents. The relative abundance of the products with FQ usually followed a binomial distribution, which suggests that the labeling sites were uniformly accessible to this reagent. In contrast, the distribution of reaction products with the Chromeo reagents did not follow a binomial distribution for reactions performed in the absence of sodium dodecyl sulfate (SDS); it appears that the protein labeled with the Chromeo reagents refolded into a relatively stable secondary structure that hid some reactive sites. The reaction with the Chromeo reagent did follow the binomial distribution if the protein underwent treatment with 1% SDS at 95 degrees C for 5 min, which apparently disrupts the protein's secondary structure and allowed uniform access to all labeling sites. Chromeo 503 labeled seven of the 13 primary amines in denatured alpha-lactalbumin.