Human small nuclear ribonucleoproteins (snRNPs) containing U snRNAs have been fractionated into three RNA-specific populations, and snRNPs containing U1 and U2 snRNAs have been isolated~by biochemical methods. U1 and U2 snRNPs remained immunoprecipitable by Systemic Lupus Erythematosus antibodies during isolation, and purified snRNPs contained polypeptides of the same molecular weights as those defined by immunoprécipitation of crude extracts. The polypeptide components of U1 and U2 snRNPs have been compared by two-dimensional gel electrophoresis and immunobinding. U1 and U2 snRNPs contained both unique and common polypeptides. Purified U1 snRNPs contained U1 RNA and 1 snRNP polypeptides of molecular weights 67, (P67), 3, (P3), 23, (P23F), 21,5 (P22), 17,5 (P18), 2 at 12,3 (P12F and P12S), 1,2 (P1), 9,1 (P9), and 8,5 (P8). Purified U2 snRNPs contained U2 RNA and 9 snRNP polypeptides including the common polypeptides P23F, P22, P12F, P12S, P1, P9, and P8 as well as two U2 specific polypeptides of 23, (P23S) and 27, (P27) daltons. Five of the common polypeptides, including P23F, P22, P12F, P12S, and P9, were basic and may be important in snRNP assembly. One common polypeptide, P1 was acidic, and the remaining common polypeptide, P8, was neutral. Two of the U1 specific polypeptides, P3 and P18, as well as the two U2 specific polypeptides, P27 and P23S, were neutral. The third Ul specific polypeptide, P67, was slightly basic. Two of the common polypeptides, P23F and P22, were present in nonstoichiometric amounts and were recognized by a monoclonal anti-Sm antibody. Only one of them is present in rodent cells (Conner et al., 1982). Thus, in human cells, P23F and P22 may be two variants of the same polypeptide, with each snRNP complex containing either P23F or P22. P12F is also recognized by the same monoclonal antibody, suggesting it may be related to P23F and P22. Because Ul and U2 snRNPs contain common and unique polypeptides, it is suggested that they serve similar but distinct functions in. vivo. Ul has been implicated in hnRNA splicing (Lerner et al., 198; Rogers and Wall, 198). U2 may be involved in another aspect of hnRNA processing.