In 1956 the first successful prototype of an Aerosol Spectrometer was built and used subsequently for the size analysis of synthetic and natural smog. A significantly improved type was available during the period of this contract. Its performance has been studied - particularly with regard to the possibility of tracing with it associated states between gas components and air-borne particles. This subject forms Part A of the report. The Bacteriological Irritation Analogue (BIA) test, its instrumentation and application is described in Part B. Attached to the report is an Appendix, containing a theoretical analysis and interpretation of recent animal tests in synergistic aerosols. (Author Modified Abstract)