A group of penicillins have been tested for their effect on the sensitivity of limulus amebocyte lysate test (LAL). Cloxacillin, cephalothin and cefuroxime inhibited the gel formation at concentrations above 2 mg mL-1, while ampicillin, methicillin and mecillinam showed no inhibitory effects upto concentrations of 10 mg mL-1. For benzylepenicillin the maximum non-inhibitory concentration was 10,000 units mL-1. However, the dilutions required to overcome inhibition were within the limits of maximum valid dilutions computed for each product showing that LAL test is valid for these products at their non-inhibitory concentrations.