Previous research has found that "similar" sounds interact in phonological nasal consonant harmony, wherein certain consonants become nasals when the word contains a nasal (e.g., Kikongo: /-kun-idi/ [right arrow] [-kun-ini] "planted"). Across languages, stops and approximants are chiefly affected, especially voiced consonants and ones that match in place of articulation with the nasal. Three experiments investigated whether a parallel occurs in consonants showing greater likelihood to interact in speech errors with nasals. The experiments, which elicited errors using the SLIPS technique with English speakers, revealed the following asymmetries in consonants' participation in errors with nasals: (i) voiced stops ("b, d") greater than voiceless stops ("p, t"), (ii) voiced stops with same place greater than voiced stops with different place, and (iii) approximants ("r, l") greater than voiceless stops ("p, t"). These correlate with preferentially affected segments in nasal consonant harmony. The data support a uniform phonological similarity scaling for nasal-oral consonants across phonological harmony processes and speech errors. Further, they are consistent with theoretical proposals that consonant harmony has functional origins in facilitating language production. (Contains 1 figure, 8 tables and 18 footnotes.)