RECENTLY, we saw two patients in whom a heavy concentration of urinary protein reported from the laboratory was eventually proved to be a false-positive result, representing not protein but nafcillin excreted in the urine. Report of Cases Case 1.— A 13-year-old boy with staphylococcal osteomyelitis of the left femur was treated intravenously with 12 gm of nafcillin sodium daily. Shortly thereafter his urine showed 4+ protein reaction on routine analysis with sulfosalicylic acid. Renal function, as measured by blood urea nitrogen (BUN) value, serum creatinine level, and creatinine clearance, was within normal limits. Protein level in a 24-hour urine sample (measured by the standard trichloroacetic acid method) was 3.9 gm. Results of urine-sediment examination were unremarkable, showing no free fat, oval fat bodies, or fatty casts. Subsequent protein testing on several occasions with sulfosalicylic acid again revealed 3+ to 4 + protein reaction, but a dipstick specific for albumin gave a